In addition to invoking Catch2's test runner after Minetest's homemade
runner, this refactors the tests to follow the DRY principle, and gives
them expressive names and clear assertions. Catch2 is already bundled
with Minetest, so there are no added dependencies.

This switches infostream to rawstream so that test runner output is
displayed, and returns the correct boolean depending on the results. The
tests are now found by setting the configuration instead of invoking the
command line parser.

Co-Authored-By: Lars Müller <appgurulars@gmx.de>

It's necessary to flush stderr before printing to stdout in adition to
flushing stdout before printing to stderr, to make sure all output is
ordered correctly.