CI: speedup POT3D tests

Currently the POT3D tests runs take minutes. Instead, they should take seconds, so we need about 60x speedup of the runtime. To do that, we need to device a testcase that is smaller.

That way we can focus on compiling POT3D in many different ways via LFortran, and spend CI time on that, and not waste it actually running POT3D. We only need to run POT3D to check that it was compiled correctly, and we can do that with a smaller test case.