Run unstable unit tests in CI

Running unstable unit tests in CI should help with making sure they
don't fail permanently without the fact being noticed in daily
pipelines.

(cherry picked from commit 1d8788464e)

.gitlab-ci.yml

@@ -676,6 +676,7 @@ unit:gcc:buster:amd64:
       artifacts: true
 
 # Jobs for regular GCC builds on Debian 11 "bullseye" (amd64)
+# (The second unit test job also executes unstable unit tests.)
 
 gcc:bullseye:amd64:
   variables:
@@ -699,6 +700,20 @@ unit:gcc:bullseye:amd64:
     - job: gcc:bullseye:amd64
       artifacts: true
 
+unit:gcc:bullseye:unstable:amd64:
+  <<: *debian_bullseye_amd64_image
+  <<: *unit_test_job
+  variables:
+    CI_ENABLE_ALL_TESTS: 1
+  needs:
+    - job: gcc:bullseye:amd64
+      artifacts: true
+  only:
+    - api
+    - schedules
+    - triggers
+    - web
+
 # Jobs for cross-compiled GCC builds on Debian 11 "bullseye" (amd64) with
 # 32-bit libraries