Run the tests group-by-group

Instead of running one big test suite, split it up into the test groups.

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.