Added jtreg-buffer external suite

[judovana]

This PR is adding our buffer of not-yet-if-ever upstreamed jtreg tests, as another external testsuite. It do pass locally, however in grinder it fails https://ci.adoptium.net/view/Test_grinder/job/Grinder/9229/console Error: cannot find the following tests: jtreg-buffer (note: group target such as sanity is not accepted inside testList) So i guess there is somwehre some another protection.

Except that issue, for which I do not have cure, few questions

• external tests do not run with podman-docker docekr wrapper. The traces are quite cryptic, is this already tracked?
  • Is there generic/general opinion on moving to root-less container engine (thus podman)?
• on grinder jobs, the external tests run as root? I had not found a trace of change of user in logs, so am wondering
  • local root-less run proiperly failed for me
• is there any way how to copy any testsuite results from container to host?
• the tap file is correctly on host, but the suite itself is generating jtrfiles and jtr.xml which I would llike to be able to archive on demand if there occure issues.