ci: Enable coverage reports in test build

While we don't have an high number of tests, we still have some code
coverage and so we can track this via gitlab CI, given that it supports it
natively.

So add gcovr to the DockerFile dependency, build with -Db_coverage=true
meson native parameter, and add another manual job to make ninja to generate
the coverage reports on requests or in any master or tag ref.

Keep the artifacts around to be able to browse the generated HTML files and
eventually print the text reports so that they can be parsed by gitlab.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1236

.gitlab-ci.yml

@@ -4,6 +4,7 @@ stages:
  - review
  - build
  - test
+ - coverage
 
 check-commit-log:
   stage: review
@@ -17,7 +18,7 @@ check-commit-log:
 build-mutter:
   stage: build
   script:
-    - meson . build -Dbuildtype=debugoptimized -Degl_device=true -Dwayland_eglstream=true --werror --prefix /usr
+    - meson . build -Dbuildtype=debugoptimized -Db_coverage=true -Degl_device=true -Dwayland_eglstream=true --werror --prefix /usr
     - ninja -C build
     - ninja -C build install
   artifacts:
@@ -74,10 +75,30 @@ test-mutter:
     - >
       dbus-run-session -- xvfb-run -s '+iglx -noreset'
       meson test -C build --no-rebuild -t 10 --verbose --no-stdsplit --print-errorlogs --wrap catchsegv
+  artifacts:
+    expire_in: 1 day
+    paths:
+      - build
   only:
     - merge_requests
     - /^.*$/
 
+test-mutter-coverage:
+  stage: coverage
+  dependencies:
+    - test-mutter
+  script:
+    - ninja -C build coverage
+    - cat build/meson-logs/coverage.txt
+  artifacts:
+    paths:
+      - build/meson-logs
+  when: manual
+  except:
+    refs:
+      - tags
+      - master
+
 can-build-gnome-shell:
   stage: test
   dependencies:

.gitlab-ci/Dockerfile

@@ -16,7 +16,7 @@ RUN dnf -y update && dnf -y upgrade && \
 
     # For running unit tests
     dnf install -y xorg-x11-server-Xvfb mesa-dri-drivers dbus dbus-x11 \
-        '*/xvfb-run' gdm-lib accountsservice-libs gnome-control-center \
+        '*/xvfb-run' gdm-lib accountsservice-libs gnome-control-center gcovr \
         --setopt=install_weak_deps=False && \
 
     # GNOME Shell