Set collation for postgresql test DBs.

[simi]

I was facing test failures related to different order than expected since I have lc_collate set to cs_CZ.UTF-8.

I propose to set lc_collate on test DB creation to en_US.UTF-8. Similar strategy is used also for MySQL at

rails/activerecord/Rakefile

Lines 223 to 224 in a01d6f8

	%x( mysql #{connection_arguments["arunit"]} -e "create DATABASE #{config["arunit"]["database"]} DEFAULT CHARACTER SET utf8mb4" )
	%x( mysql #{connection_arguments["arunit2"]} -e "create DATABASE #{config["arunit2"]["database"]} DEFAULT CHARACTER SET utf8mb4" )

.

example failure (fixed by this PR)

Using postgresql                                                                                                                                                                   [2260/7680]
Run options: --seed 10882                                                                                                                                                                     
                                                                                                                                                                                              
# Running:                                                                                                                                                                                    
                                                                                                                                                                                              
.................................F                                                                                                                                                            
                                                                                                                                                                                              
Failure:                                                                                                                                                                                      
CalculationsTest#test_pluck_loaded_relation_sql_fragment [test/cases/calculations_test.rb:1081]:                                                                                              
--- expected                                                                                                                                                                                  
+++ actual                                                                                                                                                                                    
@@ -1 +1 @@                                                                                                                                                                                   
-["37signals", "Apex", "Ex Nihilo"]                                                                                                                                                           
+["Apex", "Ex Nihilo", "Ex Nihilo Part Deux"]                                                                                                                                                 
                                                                                                                                                                                              
                                                                                                                                                                                              
                                                                                                                                                                                              
rails test test/cases/calculations_test.rb:1078                                                                                                                                               
                                                                                                                                                                                              
......................................................................................................S..F                                                                                    
                                                                                                                                                                                              
Failure:                                                                                                                                                                                      
CalculationsTest#test_pick_loaded_relation_sql_fragment [test/cases/calculations_test.rb:1138]:                                                                                               
Expected: "37signals"                                                                                                                                                                         
  Actual: "Apex"                                                                                                                                                                              
                                                                                                                                                                                              
                                                                                                                                                                                              
rails test test/cases/calculations_test.rb:1135                                                                                                                                               
                                                                                                                                                                                              
....................................................                                                                                                                                          
                                                                                                                                                                                              
Finished in 1.409349s, 136.2331 runs/s, 366.1265 assertions/s.                                                                                                                                
192 runs, 516 assertions, 2 failures, 0 errors, 1 skips