While we might never be able to have 100% of our code strict, we can at
the very least do it for all of our tests. This ensures that our tests
are using our API with the types as intended by the test author, and not
silently be cast to what our API requires.
See #1285, which went stale and was closed, but recently received some positive feeback. It seems easy to implement, and the only other plaintext file format, Html, allows loading from a string. Those two reasons combined suggest that we should do it.