Allow the formatting test to fail

By allowing to to fail, we won't block the rest
of the CI tests, and also won't block the merge.

.gitlab-ci.yml

@@ -14,6 +14,7 @@ before_script:
 # https://gitlab.com/gitlab-org/gitlab/issues/12850
 test_formatting:
   stage: test
+  allow_failure: true
   script:
     - echo "Testing formatting from commit ${MERGE_BASE_SHA}"
     - ./tools/check_coding.sh --diff --ci --commit ${MERGE_BASE_SHA}