"Long" tests

[smartalecH]

Now that fmmax has some updated vector field formulations, it would be good to rerun some longer, more exhaustive tests just to ensure things are as we expect. Here are some of the axes we should check:

• All of the different vector field formulations (currently the tests includes mostly POL)
• Complicated geometries with metals and metasurfaces (e.g. modify the uLED to include a random metasurface)
• Convergence checks with increased Fourier terms and BZ points

By specifying a separate set of "long" (or expensive) tests, we don't have to run them with CI. Rather, we can just run them locally when a major change is made. If we see regressions, we can manually git bisect to find the root of the issue.