Remove `TestCask` class.

Library/Homebrew/cask/lib/hbc/cask_loader.rb

Library/Homebrew/cask/spec/spec_helper.rb

Library/Homebrew/cask/test/cask/cli/cat_test.rb

Library/Homebrew/cask/test/test_helper.rb

.../cask/Casks/appcast-checkpoint-sha256-for-empty-string.rb

...support/fixtures/cask/Casks/appcast-invalid-checkpoint.rb

...support/fixtures/cask/Casks/appcast-missing-checkpoint.rb

...t/support/fixtures/cask/Casks/appcast-valid-checkpoint.rb

.../test/support/fixtures/cask/Casks/appdir-interpolation.rb

...Homebrew/test/support/fixtures/cask/Casks/auto-updates.rb

...Homebrew/test/support/fixtures/cask/Casks/bad-checksum.rb

...y/Homebrew/test/support/fixtures/cask/Casks/basic-cask.rb

...Homebrew/test/support/fixtures/cask/Casks/container-7z.rb

...omebrew/test/support/fixtures/cask/Casks/container-air.rb

...ebrew/test/support/fixtures/cask/Casks/container-bzip2.rb

...omebrew/test/support/fixtures/cask/Casks/container-cab.rb

...omebrew/test/support/fixtures/cask/Casks/container-dmg.rb

...mebrew/test/support/fixtures/cask/Casks/container-gzip.rb

...mebrew/test/support/fixtures/cask/Casks/container-lzma.rb

...omebrew/test/support/fixtures/cask/Casks/container-pkg.rb