Make the unit test a Swift Package

[uhooi]

Issue

• close #281
• close #207

Details

• Split unit tests
• Remove Slather

References

• https://stackoverflow.com/questions/9245764/how-do-you-move-an-xcode-4-2-project-file-to-another-folder
• https://marycore.jp/prog/xcode/directory-not-found-for-option-f/