add support to run test suite at the end of a build job

[trz42]

• Adds the necessary steps to the Slurm job script.
• Adds handling of the result of the test suite run (i.e., updating the PR comment).
• Adds one config setting job_test_unknown_fmt and corresponding description.

Also see accompanying PR for software-layer EESSI/software-layer#366

edit (by @boegel): it's worth noting that these changes will require adding job_test_unknown_fmt to the bot configuration file, or the job manager will crash with:

...
  File "/home/boegel/eessi-bot-software-layer/eessi_bot_job_manager.py", line 572, in process_finished_job
    job_test_unknown_fmt = finished_job_comments_cfg[JOB_TEST_UNKNOWN_FMT]
  File "/usr/lib64/python3.6/configparser.py", line 1233, in __getitem__
    raise KeyError(key)
KeyError: 'job_test_unknown_fmt'

when processing a finished job

[boegel]

testing this via boegel/software-layer#30 ...

[boegel]

Extensively tested via boegel/software-layer#30, good to merge, thanks @trz42!