Use feature type for meson options

This could break anyone currently using -Dunit_tests=false, but probably
not many people choose that.


(cherry picked from commit f2b8170e)
1 job for gnome-3-32 in 4 minutes and 38 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #226537
flatpak

00:04:38