When running on non-Apple platforms, report test failures using XCTestCase.recordFailure()

[younata]

Currently, if you're running Nimble on non-Apple platforms, we report test failures using XCTFail. Which works, but is a subpar experience.

Let's instead use what Quick uses to report errors and use XCTestCase.recordFailure().

[younata]

After I looked in to this for about a minute, I remembered why Nimble uses XCTFail on non-apple platforms: There isn't an API to grab the current test case, and there isn't an obviously good place for Nimble to register its testcasetracker like there is in Objective-C environments.

Filed swiftlang/swift-corelibs-xctest#491.