Add swift-testing-expectation

[dfed]

• Project Name: swift-testing-expectation
• Project URL: https://github.com/dfed/swift-testing-expectation
• Project Description: Create an asynchronous expectation in Swift Testing.
• Why it should be added to awesome-swift: Useful tool for folk migrating to Swift Testing. Also useful for folk who are writing new Swift Testing tests
• At least 15 stars (GitHub project)
• Support Swift 5
• Updated contents.json instead of README
• Lib is fully open sourced, written in Swift and not a wrapper over compiled lib
• Description does not say "written in Swift" or variant 🤓

Last checkmark is dubious, but I'm I'm hoping that "Swift Testing" (emphasis added) gets me a pass since this is explicitly built on top of the swift-testing framework.

[vpeschenkov]

@dfed thanks!