oeqa/selftest/bbtests: clean up local DL_DIR/SSTATE_DIR safely
Now that the selftests clean selftest.inc in the tearDown using bitbake in a
function passed to addCleanup (which happens after teardown) will use the
default configuration and not the customised one.
As the intention was to clean away the temporary DL_DIR and SSTATE_DIR, simple
use track_for_cleanup to delete the entire directory instead.
Signed-off-by: Ross Burton <ross.burton@intel.com>
