meson: run the test suite per group

We can't run this in parallel so it doesn't gain us any speed advantage. If
anything, it'll be slower because it's more setup time in between. But: meson
doesn't display the result until the test suite finished, so having this
broken up into smaller chunks means we're more likely to see a general failure
early.

And the failure should be quicker to reproduce as well.

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