Disable OpenBSD system test parallelism in CI

Make OpenBSD system tests more stable by disabling parallelism in CI.

Accordingly, they will run 3-4 times longer.

(cherry picked from commit d73eba57bd)

.gitlab-ci.yml

@@ -1225,9 +1225,11 @@ system:clang:openbsd:amd64:
   <<: *api_schedules_triggers_web_triggering_rules
   variables:
     USER: gitlab-runner
+    TEST_PARALLEL_JOBS: 1
   needs:
     - job: clang:openbsd:amd64
       artifacts: true
+  timeout: 2h
 
 unit:clang:openbsd:amd64:
   <<: *openbsd_amd64_image