Make sure we have avocado_ + __name__ prefixes to the dirs
created with tempfile, so that we have an easy way to spot
when we are not cleaning up directories properly. This mostly
concerns unittests, although the kernel_build lib and the
iso9660 lib also uses that API.
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>