[SWT-NNNN] Exit tests

One of the first enhancement requests we received for swift-testing was the
ability to test for precondition failures and other critical failures that
terminate the current process when they occur. This feature is also frequently
requested for XCTest. With swift-testing, we have the opportunity to build such
a feature in an ergonomic way.

Read the full proposal [here](https://github.com/apple/swift-testing/blob/jgrynspan/exit-tests-proposal/Documentation/Proposals/NNNN-exit-tests.md).

Author: grynspan