Add postgresql_timestamp test step

Tests Active Record using Postgres with `timestamp` (not `timestamptz`) as the default datetime column type. rails/rails#41084 adds the relevant task, so that needs to be merged first.

pipeline-generate

@@ -185,16 +185,17 @@ end
 
 # GROUP 1: Runs additional isolated tests for non-PR builds
 %w(
-  actionpack      test                default
-  actionmailer    test                default
-  activemodel     test                default
-  activesupport   test                default
-  actionview      test                default
-  activejob       test                default
-  activerecord    mysql:test          mysqldb
-  activerecord    mysql2:test         mysqldb
-  activerecord    postgresql:test     postgresdb
-  activerecord    sqlite3:test        default
+  actionpack      test                        default
+  actionmailer    test                        default
+  activemodel     test                        default
+  activesupport   test                        default
+  actionview      test                        default
+  activejob       test                        default
+  activerecord    mysql:test                  mysqldb
+  activerecord    mysql2:test                 mysqldb
+  activerecord    postgresql:test             postgresdb
+  activerecord    postgresql_timestamp:test   postgresdb
+  activerecord    sqlite3:test                default
 ).each_slice(3) do |dir, task, service|
   next if task == "mysql:test" && RAILS_VERSION >= Gem::Version.new("5.x")