Enable simultaneous test runs by interpolating TEST_ENV_NUMBER into s…

…implecov's dir name

Otherwise, the test runners overwrite each other's coverage data, producing false negatives.

TEST_ENV_NUMBER is the variable used in parallel_tests for database names, etc.

test/helpers/start_simplecov.rb

@@ -13,6 +13,7 @@ def start_simplecov
         minimum_coverage 100 if this_process_responsible_for_coverage_reporting?
         command_name SIMPLECOV_COMMAND_NAME if child_test_process?
         formatter simplecov_formatter_class
+        coverage_dir "coverage/#{ENV['TEST_ENV_NUMBER']}" if !ENV['TEST_ENV_NUMBER'].nil?
       end
     end