ci: add test coverage to the GitLab UI

This is a small nit, but allows to see the coverage percentage change
across MRs. It gives a small dopamine boost to see an MR you sent has
tests and made a small impact in the overall coverage :)

Part-of: https://gitlab.postmarketos.org/postmarketOS/pmbootstrap/-/merge_requests/2596

.gitlab-ci.yml

@@ -52,6 +52,7 @@ pytest:
   after_script:
     - "cp /home/build/.local/var/pmbootstrap/log_testsuite.txt ."
     - "cp /home/build/.local/var/pmbootstrap/log.txt ."
+  coverage: '/TOTAL.*? (100(?:\.0+)?\%\|[1-9]?\d(?:\.\d+)?\%)$/'
   artifacts:
     when: always
     paths:
@@ -232,4 +233,3 @@ pkgrel_bump amd64:
   extends: .integration
   script:
     - .ci/integration_tests/bump_autobuild qemu-amd64 hello-world
-