Adds a "build_tests" command which builds all resources required for testing. It has some simple dependency awareness and only rebuilds if any of the direct sources have changed. Passing -f/--force will force a rebuild.
Adds a new "test" command which builds the main extension module in the source directory, runs "build_tests" and finally runs all tests.
Extends the "distcheck" command to run the tests after extracting the source distribution and run the distcheck command in the gitlab CI script.
Only tested on Linux.