oeqa/selftest: recipetool/devtool: Avoid load_plugin test race
This bug has plagued the autobuilder for a couple of years and we've
struggled to reproduce/debug it.
The problem is the "lib" directory in meta-poky used during the load_plugin
tests for recipetool and devtool can race and one can delete the files
from the other leading to test failures.
Deleting the lib directory only if empty will avoid this.
[YOCTO #13070]
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
