Provide Stable, Portable Alternative For CFRunLoopStop

[brianmichel]

I'm currently adding support to the Nimble framework to allow it to compile and run on Windows. As this is a testing framework it needs to deal with the run loop a fair bit.

It seems there is no alternative to CFRunLoopStop() other than a new SPI https://github.com/apple/swift-corelibs-foundation/blob/f07fa9fdc94b8d2e3edff6ff33a63992d172105e/Sources/Foundation/RunLoop.swift#L246 which is used by XCTest. Right now I'm using this to get the PR to run and test successfully, but I wanted to file an issue to track this officially.

I'd love to understand what the future of these RunLoop APIs is going to look like and how to best plan here.