Add XCTexpectFailure

[ZevEisenberg]

TCA already uses a dyanmic-overlay-esque version of XCTExpectFailure to show what tests would have failed, had exhaustive testing been enabled. This is slick, and I'd like to do something similar in a testing library that already uses xctest-dynamic-overlay. I could borrow the code from TCA, but it seems like a natural fit for this library to include it.

[stephencelis]

@ZevEisenberg We'd definitely be down for that, but haven't prioritized. The main thing to recapture is that Apple's XCTExpectFailure is a continuation and passes the value returned from the closure to the caller. We can do this too, but it'll make the code path a little more precarious (it'll have to crash if the symbol isn't found).

We'll also want to make sure it's compiled out of Linux.

Are you interested in trying to PR this?

[ZevEisenberg]

(it'll have to crash if the symbol isn't found).

Do you mean that it'll need to fatalError if the dlsym call returns nil?

Anyway, I'll put up a janky PR soon because it's easier to talk about bad code than theoretical code.

[ZevEisenberg]

PR is up: #58