CI: Add job to check spec files for trailing '_spec' part

Some time ago we added some spec files, however, these were missing the trailing '_spec' part, so these were never executed.

This commit adds a new CI job to check for this problem and to catch it earlier next time. For now, this job allows errors. Lets revisit this in the future to probably make the result fatal, that is, block a merge request based on the result.

This job is limited to changes made in the relevant spec directories, however, due to the nature GitLab works currently, if pushing new branches, this job will be executed regardless of which files were changed. However, as this job is quite "light" regarding resources, this shouldn't be of any concern.

Some tests were done via intermediate commits to ensure this works as expected.

Relates !284 (merged)