Replace Quick/Nimble with XCTest

[joemasilotti]

This PR replaces the third-party test framework Quick with the one that ships with Xcode, XCTest.

Converting from Quick/Nimble to XCTest aligns the project closer to default iOS apps. And XCTest feels a bit more at home for Rails developers using TestUnit vs. RSpec.

All tests are green, locally. But a few of the Swifter server-related ones are taking up to 30 seconds to run. Sadly, I can't seem to get GCDWebServer to work on my machine to verify if this is Swifter's fault or not.

[joemasilotti]

Update: I have everything green locally with Swifter but can't seem to get the tests to pass on CI. The requests to the embedded server keep timing out.

I'd love to rip out the embedded server stuff but I don't know of a good alternative. Would love suggestions or ideas if anyone has them!

[joemasilotti]

Forget Swifter, we're using Embassy now! And this library is (more) actively maintained.

This PR is officially ready for review. :)

[zoejessica]

@joemasilotti This is looking terrific and I'm very happy to lean on XCTest and try out Embassy which looks super clean 👌

I've left some suggestions that take the opportunity to remove try!s from the path of asserting on results. I'm not at all fussed about using try! when setting up test files from disk btw, it's just the ones being asserted on that would crash the test suite instead of reporting a failure.

I also did the same in the SessionTests file when setting up the server, since that's a third party dependency.

[zoejessica]

Super! 🚀