selftest/sstatetests: add a test for CDN sstate cache

selftest/sstatetests: add a test for CDN sstate cache

Specifically, the test checks that everything needed for building
standard oe-core images for x86_64 and arm64 is available from
the cache (with minor exceptions). Going forward, a complete
world check could be enabled and additional configurations,
but that requires improvements to performance of hash equivalence
server in particular.

RP: I've disabled the tests by default so we can merge them. We will
make them live once we get to the bottom of the failures.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>