ci: Put coverage regexp into CI description

One can set it in the project, but that doesn't get copied to forks. And
that means the coverage information isn't printed in MRs sometimes.

Just add it into .gitlab-ci.yml so that it always works.

.gitlab-ci.yml

@@ -69,6 +69,7 @@ test:
     paths:
       - _build/meson-logs
     expire_in: 1 week
+  coverage: '/^TOTAL.*\s+(\d+\%)$/'
 
 test_valgrind:
   stage: test