TT#111150 Rename GitHub shellcheck test workflow to shellcheck.yml

Rename the workflow, as this is only in charge of running the shellcheck
tests. Having a workflow per checker also makes them more modular, and
easier to reuse in other projects by simply copying the files around.
And makes it easier to see what checks are currently available from
the filenames.

Rename the job to "test", for correctness, as it has implicit ordering
and meaning within the GitHub action built-in stages.

Change-Id: I3987fc03111d94a8bc64e9103e43cb4d05925e12