oeqa/concurrenttest: Use ionice to delete build directories
Autobuilder type infrastructure can benefit from deletion of certain files as
background IO due to the way Linux filesystem priority works.
We have problems where build directories as part of oe-selftest being
delete starves the running tasks of IO to the point builds take much
longer to compelte.
Having this option of running the deletion at "idle" helps a lot with
that. Use the new option added to bb.utils.prunedir().
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
