22106 FIX job: report a running file without a usable start time

mk-job versions shipped with Checkmk 2.4.0 and older determine the start time
of a job with perl and write an empty one if perl is not installed - on a
minimal RHEL 8/9 installation, for example. Such a ".<pid>running" file kept
its service before this series and lost it in "stop promoting leftover running
files over the completed job"; it now gets it back, as a WARN, and the
completed and running jobs are reported as before.

The shape test for a leftover file goes with it. A ".<pid>running" file
written by a supported agent holds exactly one line - the start time - whether
the job is still running or was killed before mk-job could remove the file,
so there is nothing in the file to tell the two apart. Only agents predating
werk 15450 wrote completion data into it, and those are outside the two
versions of backward compatibility we keep. On a supported agent, a leftover
file is mk-job's own to clean up, which cleanup_running_files does on the next
run of the same job by checking the pid.

The unit tests keep the pre-2.3 sections: what such a file describes is now
taken at face value, stale start time and all, and that is worth pinning.

CMK-35333

Change-Id: Ib94a0bc4e9be0af709b8cc8085e88e547f8a5afa