testsuite: Don't try to run tests with missing libraries

As noticed by sgraf, we were still running reqlib tests, even if the
library was not available. The reasons for this were not clear to me as
they would never work and it was causing some issues with empty stderr
files being generated if you used --test-accept.

Now if the required library is not there, the test is just skipped, and
a counter increased to mark the fact.

Perhaps in the future it would be nicer to explicitly record why certain
tests are skipped. Missing libraries causing a skip is a special case
at the moment.

Fixes #20005