tests: allow to skip check-local-gitlab-ci via "$NMTST_SKIP_CHECK_GITLAB_CI"

This will be used on our gitlab-ci tests. We do have a separate,
dedicated test on gitlab-ci for checking that "gitlab-ci.yml" is
correct.

We may intentionally want to violate that condition, but then our
`make check` should not fail.

Add a way to skip that test.

tools/check-gitlab-ci.sh

@@ -2,6 +2,10 @@
 
 set -e
 
+if [ "$NMTST_SKIP_CHECK_GITLAB_CI" = 1 ]; then
+    exit 0
+fi
+
 if [ $# -eq 0 ] ; then
     BASEDIR="$(dirname "$(readlink -f "$0")")/.."
 elif [ $# -eq 1 ] ; then