mtest: raise informative error message when test program doesn't exist

If --no-rebuild is used, the test program might not exist, spawning a
FileNotFoundError inside a long traceback rooted in subprocess.Popen
trying to run that test program. Current versions of Meson even say it's
an "unhandled python exception".

But we can do one better and actually tell the user what is wrong, why,
and what to do to fix it. And we can do so before getting waist deep in
partially running tests.

Fixes #10006

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