CTest follow-up: split fast/numerics tests and slow/physics tests

[EZoni]

Cc: @ax3l

Discussed offline with @roelof-groenewald, @oshapoval and to be discussed further.

We could introduce separate GitHub Actions workflows for fast/numerics tests and slow/physics tests, with the latter running more seldomly than the former (that is, not at every commit pushed).

Alternatively, we could come up with another solution along these lines to handle slow tests in a reasonable way without necessarily getting rid of them.