ci: Use junit test reports

They can be used to show test failure logs directly in the MR's
pipelines page.

.gitlab-ci.yml

@@ -69,6 +69,8 @@ test:
     - ninja -C _build coverage
     - cat _build/meson-logs/coverage.txt
   artifacts:
+    reports:
+      junit: "_build/meson-logs/testlog.junit.xml"
     expose_as: 'Coverage Report'
     when: always
     paths:
@@ -87,6 +89,8 @@ test_valgrind:
     - ninja -C _build
     - meson test -C _build --print-errorlogs --no-stdsplit --setup=valgrind
   artifacts:
+    reports:
+      junit: "_build/meson-logs/testlog.junit.xml"
     expose_as: 'Valgrind test logs'
     when: always
     paths: