Update the TEST_PARALLEL_JOBS value in CI

The authoritative source for this value is in the project's CI/CD
Variables Setting. The reason to keep it in .gitlab-ci.yaml as well is
to have functional testing in forks without the need to manually specify
this variable in Settings.

The tests have been executed with 4 jobs for some time now. This
"change" only brings .gitlab-ci.yaml file up to date, it doesn't
actually change the number of jobs we currently use to test.

Author: nicki-krizek

.gitlab-ci.yml

@@ -8,8 +8,10 @@ variables:
   CCACHE_DIR: "/ccache"
 
   GIT_DEPTH: 1
+
+  # The following values may be overwritten in GitLab's CI/CD Variables Settings.
   BUILD_PARALLEL_JOBS: 6
-  TEST_PARALLEL_JOBS: 6
+  TEST_PARALLEL_JOBS: 4
 
   CONFIGURE: ./configure
   CLANG_VERSION: 15