selftest/sstatetests: move exception list for cache objects to the parent class

This is useful not just in cdn/local cache tests, but also in the build bundle test,
and so should be applied by default.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>