This is needed if we compile StdlibUnittest with -sil-serialize-all
So far I added the imports only in files which needed them. But this may change, depending on the optimizer (inlining).
Adding them in all files doesn't harm and avoids confusion if someone makes an unrelated change which would result in such a linker error.
The current validation tests for the stdlib XCTest only check that it
compiles, not that it works at runtime. Begin adding tests that verify
its behavior at runtime, beginning with:
- A test that Objective-C exceptions are captured and reported.
- Tests for some function overloads of `XCTAssertEqual()`.
