Move GForce tests to own script

[MichaelChirico]

Current structure is pretty innavigable -- it would be much easier to have all the GForce tests in one place. Then we can cycle through all the optimization levels one single time and add new tests in each optimization level as needed.

[ben-schwen]

Moreover, we should add consistency tests that all optimization levels really produce the same output.

This is already demanded by commit 819ac6f but currently not always enforced.

TODO: Tests involving GForce functions needs to be run with optimisation level 1 and 2, so that both functions are tested all the time.