Separate `Homebrew::TestCase` from `testing_env`.

Library/Homebrew/.rubocop_todo.yml

Library/Homebrew/test/support/helper/fs_leak_logger.rb

Library/Homebrew/test/support/helper/test_case.rb

Library/Homebrew/test/support/helper/version_assertions.rb

Library/Homebrew/test/testing_env.rb