ci: Run coverage in all tests, and merge results

Instead of having different coverage outputs for different architectures
and compilation flags, have each of those tests run coverage in order
to generate a JSON report, and have that merged at a final common job.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2315>

.gitlab-ci.yml

@@ -291,6 +291,13 @@ build-without-native-backend-and-wayland@x86_64:
   script:
     - dbus-run-session -- xvfb-run -a -s "$XVFB_SERVER_ARGS"
         ./.gitlab-ci/run-meson.sh test -C build --no-rebuild -t 10
+  after_script:
+    - pushd build
+    - gcovr --root=..
+        --filter='\.\./src/' --filter='\.\./clutter/' --filter='\.\./cogl/'
+        --exclude='\.\./build/.*\.[ch]$' --exclude='.*/tests/.*\.[ch]$'
+        --json --output=../coverage-${CI_JOB_NAME}.json
+    - popd
   artifacts:
     expire_in: 1 day
     reports:
@@ -299,6 +306,7 @@ build-without-native-backend-and-wayland@x86_64:
     when: always
     paths:
       - build
+      - coverage-*.json
 
 test-mutter@x86_64:
   extends:
@@ -314,30 +322,21 @@ test-mutter@aarch64:
   needs:
     - build-mutter@aarch64
 
-.test-mutter-coverage:
+coverage:
   extends:
     - .fdo.distribution-image@fedora
+    - .mutter.fedora:35@x86_64
   stage: analyze
   script:
-    - ninja -C build coverage
-    - cat build/meson-logs/coverage.txt
+    - mkdir coveragereport
+    - gcovr --add-tracefile coverage-*.json
+        --html-details --print-summary --output coveragereport/index.html
   artifacts:
     paths:
-      - build/meson-logs/coveragereport
-  coverage: '/^TOTAL.*\s+(\d+\%)$/'
-
-test-mutter-coverage@x86_64:
-  extends:
-    - .test-mutter-coverage
-    - .mutter.fedora:35@x86_64
+      - coveragereport
+  coverage: '/^lines: (\d+\.\d+\%)/'
   needs:
     - test-mutter@x86_64
-
-test-mutter-coverage@aarch64:
-  extends:
-    - .test-mutter-coverage
-    - .mutter.fedora:35@aarch64
-  needs:
     - test-mutter@aarch64
 
 can-build-gnome-shell@x86_64: