Require `integration_command_test_case` in `testing_env`.

...brew/test/support/helper/integration_command_test_case.rb

Library/Homebrew/test/test/ENV_test.rb

Library/Homebrew/test/test/analytics_test.rb

Library/Homebrew/test/test/bottle_test.rb

Library/Homebrew/test/test/bundle_test.rb

Library/Homebrew/test/test/cache_formula_test.rb

Library/Homebrew/test/test/cache_test.rb

Library/Homebrew/test/test/cask_test.rb

Library/Homebrew/test/test/cat_test.rb

Library/Homebrew/test/test/cellar_formula_test.rb

Library/Homebrew/test/test/cellar_test.rb

Library/Homebrew/test/test/cleanup_test.rb

Library/Homebrew/test/test/command_test.rb

Library/Homebrew/test/test/commands_test.rb

Library/Homebrew/test/test/config_test.rb

Library/Homebrew/test/test/create_test.rb

Library/Homebrew/test/test/custom_command_test.rb

Library/Homebrew/test/test/deps_test.rb

Library/Homebrew/test/test/desc_test.rb

Library/Homebrew/test/test/doctor_test.rb