TestFoundation: Add infrastructure to assert that a test should crash
TestFoundation: Add infrastructure to assert that a test should crash

You can now define tests as follows:

```
func testCrashes() {
     assertCrashes {
          fatalError()
     }
}
```

These tests will spawn a child process and will assert that the child process crashes while executing the code in the block.
apple/swift-corelibs-foundation,e78l/swift-corelibs-foundation,e78l/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,e78l/swift-corelibs-foundation,e78l/swift-corelibs-foundation,e78l/swift-corelibs-foundation