testimage: implement test artifacts retriever for failing tests

testimage: implement test artifacts retriever for failing tests

Add a basic artifacts retrievers in testimage class which:
- triggers when at least one runtime test fails but tests execution
  encountered no major issue
- reads a list of paths to retrieve from TESTIMAGE_FAILED_QA_ARTIFACTS
- checks for artifacts presence on target
- retrieve those files over scp thanks to existing ssh class
- store those files in an "artifacts" directory in "tmp/log/oeqa/<image>"

This implementation assumes that the SSH or Qemu target has run and
finished gracefully. If tests do not finish because of an exception,
artifacts will not be retrieved

Bring partial solution to [YOCTO #14901]

Signed-off-by: Alexis Lothoré <alexis.lothore@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>