Failing tests when gfortran is not available

While working on #414 and debugging some failing tests, I realised that we have at least 13 tests that require gfortran to be available (or maybe any other compiler, I didn't check the details).

We have gfortran in our CI, so we don't see that problem. But when I artificially disabled gfortran (install a failing gfortran script in your `$PATH` :)), I get 13 test failures.

Ideally, all these tests should be fixed to use compiler stubs (which can mark them as being available).

Start by running the test suite with gfortran unavailable, as described in the issue, and identify the 13 failing tests. Inspect how those tests detect compiler availability and replace their dependency on a real compiler with compiler stubs. Done means the affected tests pass when gfortran is unavailable.

Written by the indexing model from the issue text.