Make `XCTMain()` return its exit code instead of calling `exit()`.

[grynspan]

Right now, XCTMain() returns Never and calls exit() to report its status. This makes it difficult (if not outright impossible) to do any post-test cleanup work like writing an event log or other cleanup.

This PR adds a new overload of XCTMain() that instead returns the intended exit code of the process (as CInt, since process exit codes passed to exit() are always of type int in C.) The new overload is, for now, disfavoured until we can teach SwiftPM about it. Once we can (in a subsequent PR), we'll mark the old one disfavored and deprecated.

[grynspan]

@swift-ci please test

[grynspan]

@swift-ci please test

[grynspan]

@swift-ci please test Windows

[grynspan]

Windows build failure is unrelated.

[grynspan]

@swift-ci please test Windows