Allow specifying NEEDLES_DIR to be relative to CASEDIR

[Martchus]

• Allows prepending %CASEDIR% to relative NEEDLES_DIR so %CASEDIR% is replaced with the path where the custom CASEDIR will be checked out
• Enables use-case of specifying a custom CASEDIR and a custom NEEDLES_DIR where the custom needles dir is a relative path within the custom CASEDIR (and not a separate repository or a relative path within the default needles directory).
• See https://progress.opensuse.org/issues/117133
• See NEEDLES_DIR documentation and/or behaviour could be improved #4576

[codecov]

Codecov Report

Merging #4851 (cc1ec95) into master (2dda009) will not change coverage.
The diff coverage is 100.00%.

❗ Current head cc1ec95 differs from pull request most recent head 12f10ef. Consider uploading reports for the commit 12f10ef to get more accurate results

@@           Coverage Diff           @@
##           master    #4851   +/-   ##
=======================================
  Coverage   98.12%   98.12%           
=======================================
  Files         378      378           
  Lines       34985    34985           
=======================================
  Hits        34329    34329           
  Misses        656      656           

Impacted Files	Coverage Δ
lib/OpenQA/Worker/Engines/isotovideo.pm	97.22% <100.00%> (+0.05%)	⬆️
t/24-worker-engine.t	100.00% <100.00%> (ø)
t/14-grutasks.t	100.00% <0.00%> (ø)
lib/OpenQA/CacheService/Client.pm	100.00% <0.00%> (ø)
t/ui/16-tests_job_next_previous.t	100.00% <0.00%> (ø)

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[okurz]

Nice! Documentation please. Maybe mention in the os-autoinst backend_vars as well

[github-actions]

Great PR! Please pay attention to the following items before merging:

Files matching docs/*.asciidoc:

• Consider generating documentation locally to verify it is rendered correctly using tools/generate-htmldoc

This is an automatically generated QA checklist based on modified files

[phil-hands]

I hesitated to word the concluding sentence like that when I was trying to come up with something, because e.g. the /master/ bit of that URL isn't definitely correct under all circumstances, but other than that I think that's great, and if you are OK with being mostly right-ish rather than absolutely correct, that's fine with me too :-)

[Martchus]

You're right. I replaced the "master" part with "…".