meson: group all tests under suite names

Three suite names to allow for filtering tests: 'valgrind', 'root',
'hardware'. The latter two require root/hardware to succeed, the former labels
tests that should be run under valgrind.

Usage is documented in the docs now, but basically:

$ meson test --setup=valgrind --suite=valgrind
$ meson test --no-suite=root

This is documented a bit now and because we now rely on meson test, let's
replace all ninja test invocations with meson test instead for consistency.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>