Add a new option `SAPI_ENABLE_CONTRIB_TESTS` that builds the sandboxes
in `contrib/` if set.
The new `contrib/CMakeLists.txt` should be extended each time we add a
new sandbox to `contrib/`.
This is in preparation of a follow-up change that should test the
sandboxes using GitHub Actions.
Drive-by:
- Rename the tests in `jsonnet_tests.cc` to conform to the style guide.
PiperOrigin-RevId: 425874289
Change-Id: I5b31546ccf84444c2480287220ef88abbd78a235