Test discovery for XCTest

[Uncommon]

Create a way to run specs easily under XCTest.

[Uncommon]

Possibilities:

• Use XCTestObservation.testSuiteWillStart() as an entry point. Hopefully adding tests to the suite at this point will work.
• Like Quick does, use ObjC runtime calls to find all Rundown.TestCase subclasses and collect their test data
• Quick uses method swizzling to support running a specific test class from Xcode; see if there's another way around this
• How to create the XCTest objects?
  • init(selector:) doesn't say what object the selector will be called on.
  • init(invocation:) isn't usable in Swift because NSInvocation is unavailable; it's an ObjC runtime thing. Does this have to be done in ObjC?

[Uncommon]

Other notes on working with XCTest:

• XCTContext requires the closure to be @MainActor. Is this a potential problem for some kinds of tests?
• When running a specific test from Xcode, where does the test name/invocation come from? It doesn't seem to be a command line argument or environment variable.
• Would it be good to follow the SwiftUI pattern and have each test be the test property of a class? Is it more desirable to have multiple test methods in a class, or does the describe/context/it structure make that unnecessary?