oeqa/selftest/externalsrc: add test for srctree_hash_files

Signed-off-by: Peter Marko <peter.marko@siemens.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit 7b9728e5)
Signed-off-by: Steve Sakoman <steve@sakoman.com>