Let's make this behave more like all the rest of the meson stuff.
This also is the first step to making it a bit more flexible so we
can define integration tests in different ways as will be seen in
the next commits.
Let's make the dropin, to make the build dir writable for gcov, a bit
more generic, so it can be used by all units starting with prefix test-.
This should help with a bunch of recent reports about missing coverage I
got, as well as with existing test units using DynamicUser=true.
This might feel a bit like a magic trick from behind the curtains, but I
want to touch the actual tests as little as possible, since it makes them
unnecessarily messy (see the various workarounds for sanitizers), and
the coverage reports are generated only in a specific CI job anyway.