Use CMAKE_GUARD for tests that require MPI

This results in a consistent message explaining why the test was skipped.

Mentioned in merge request !179 (closed)

I also tested this with -DCMAKE_DISABLE_FIND_PACKAGE_MPI=TRUE and got the expected skip messages.

@joe Are you ok with this approach? If yes, I'll merge this.

@gruenich definitely OK with it. Much better approach then my original one. And seems to cover more tests. CI has passed, so I see no reason not to merge.

Mentioned in commit 6aa023e6

Status changed to merged