Have has_static_section etc. filter sections other than __static

I have a valid use case, where I want to consider static variations that are not compile definitions and therefore shouldnt reside in __static. I will not be possible to put that stuff into dune-testtools, so I will introduce some code duplication downstream. Then again, I need some minimal support within dune-testtools, namely: Have the python scripts that the CMake macros call to learn about static variations take an additional parameter specifiying which sections to filter. This parameter will of course default to __static, preserving the current behaviour.