meson: Make test output much more useful on failure (both in CI and locally)

I've gotten totally fed up with the amount of clicking I need to do
every time a test fails in CI to find out what exactly failed. The
output that meson gives in its main output is completely useless
currently. Which means I have to go click through CirrusCI its (pretty
terrible) filebrowser UI searching for log files or regression.diffs.

And locally I have the exact same problem when a perl test is failing.
Meson often shows no useful output at all, which means I have to find
some unknown log file somewhere in my build directory. And then I have
to find the useful spot in that log file that actually tells me what the
error was.

So attached are a few small patches that greatly improve the test output
of the most common test failure reasons (at least the ones I tend to run
into myself).

An example CI run with some intentional failures (attached as the
nocfbot patch) can be found here:
https://cirrus-ci.com/task/6592551433535488