[5.1 4/24] Update “unable to load standard library” test (#23990) #24735
Cherry-picks #23990 to swift-5.1-branch-04-24-2019.
Original reviewed by @jrose-apple.
This PR modifies one test to handle cases where content could be located in the SDK. It is also compatible with cases where the content is not in the SDK, and has passed in internal-master for nearly a month now. It doesn't change any shipping software at all.