testsuite: Warn if test will never be run
We currently use the test's func to determine whether it is a compile-time or a run-time test. This means that run_command tests and the like are considered to be neither run-time nor compile-time tests and only get run in the normal way. This is quite surprising (see #16042) and has meant that some tests have gone silently un-run in the past. Warn when this happens.
Frankly, this design is a bit unfortunate but changing it is a bit more work than I have time to invest at the moment.