tests: remove more per-test file system cleanup

Library/Homebrew/test/ARGV_test.rb

Library/Homebrew/test/cleanup_test.rb

Library/Homebrew/test/diagnostic_test.rb

Library/Homebrew/test/formula_installer_test.rb

Library/Homebrew/test/formula_test.rb

Library/Homebrew/test/formulary_test.rb

Library/Homebrew/test/gpg_test.rb

Library/Homebrew/test/install_test.rb

Library/Homebrew/test/keg_test.rb

Library/Homebrew/test/migrator_test.rb

Library/Homebrew/test/os/mac/keg_test.rb

Library/Homebrew/test/tap_test.rb

Library/Homebrew/test/versions_test.rb