dune-ctest now intermittantly fails for skipped tests

I ran pipeline#16502 right after !628 (merged) was merged. It tries to exercise the simd tests in all configurations -- however, on images that don't include vc the test for vc is skipped. It looks like skipped tests now sometimes lead to overall test failures.

The test failure typically looks like this:

  9/105 Test  #14: vctest .................................***Skipped   0.04 sec
[...]
99% tests passed, 1 tests failed out of 105

Label Time Summary:
quick    =  93.60 sec (105 tests)

Total Test time (real) =  34.13 sec

The following tests FAILED:
	 14 - vctest (Not Run)
Errors while running CTest
Internal error: 'NoneType' object has no attribute 'text'
--- Failed to build dune-common ---
Terminating dunecontrol due to previous errors!
ERROR: Job failed: exit code 1

The following jobs have skipped vctest and failed:

• debian:9 clang-3.8-14
• debian:9 gcc-6-14
• ubuntu:16.04 gcc-5-14

The following jobs have skipped vctest and succeeded:

• debian:10 clang-7-libcpp-17

The other jobs succeeded or failed for different reasons.