CI: add a test for checking the validity of .mr-label-maker.yml

Just run a dry-run of mr-label-maker on the current MR is sufficient
to find out bugs

Requires https://gitlab.freedesktop.org/freedesktop/mr-label-maker/-/merge_requests/37

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28331>

.gitlab-ci.yml

@@ -310,6 +310,22 @@ sanity:
     - placeholder-job
 
 
+mr-label-maker-test:
+  extends:
+    - .fdo.ci-fairy
+  stage: sanity
+  rules:
+    - !reference [.mr-label-maker-rules, rules]
+  variables:
+    GIT_STRATEGY: fetch
+  timeout: 10m
+  script:
+    - set -eu
+    - python3 -m venv .venv
+    - source .venv/bin/activate
+    - pip install git+https://gitlab.freedesktop.org/freedesktop/mr-label-maker
+    - mr-label-maker --dry-run --mr $CI_MERGE_REQUEST_IID
+
 # Jobs that need to pass before spending hardware resources on further testing
 .required-for-hardware-jobs:
   needs: