ci: Use JUnit reports from Meson

Meson has been generating Junit XML files of its test results since
0.55, so we can use that to show the test results in the GitLab UI.

.gitlab-ci.yml

@@ -250,6 +250,10 @@ test:
         ninja dist -C _build
       fi
 
+  artifacts:
+    reports:
+        junit: "_build/meson-logs/testlog.junit.xml"
+
   except:
     variables:
       - $CI_PIPELINE_SOURCE == "schedule"