Do not skip advection AMR MPI test on macOS when the upstream issue is fixed

[ranocha]

When JuliaSIMD/LoopVectorization.jl#543 is fixed, e.g., by JuliaSIMD/VectorizationBase.jl#125, we should enable the macOS MPI test disabled in #2669.

[DanielDoehring]

I noticed we currently have also long wait times for the macOS tests 🤔

[JoshuaLampert]

Yes, for some reason it takes long until the jobs actually start.