core/test.mk: Rebuild out-of-date test modules only

[dumbbell]

... instead of always rebuilding all of them each time a testsuite is executed.

The only exception is when a Makefile was modified: like for main source files, test modules are all recompiled in this case.

[dumbbell]

In the context of rabbitmq-server, time make test-build (when there is nothing to do) goes from 7 seconds to 1.2 seconds.

[essen]

I've been wondering about that as well, thanks for doing it!

[essen]

Please also add a small test to confirm it works as intended.

[dumbbell]

I added a testcase and pushed.

[dumbbell]

There are several failures on CentOS in CI that I can't reproduce locally. What do you do usually to debug such errors?

[essen]

Forgot to disable -j on the PR pipeline, CentOS is using an old Make or something and -j causes failures. Next push should succeed.

[essen]

Windows is being annoying again, I'll fix it later today.

[dumbbell]

Ok, I just force-pushed to trigger a new build.

[essen]

Alpine is a flaky test that I'm tentatively improving in the most recent commit. Anyway don't bother rebuilding.

[essen]

Just one minor comment about the test and we're good!

[essen]

Merging, thanks!