oeqa/selftest/sstatetests: re-work CDN tests, add local cache tests

oeqa/selftest/sstatetests: re-work CDN tests, add local cache tests

With the rework of printdiff, it is not longer useful for checking
absence of sstate objects in a remote http cache, as it would only
report the top level missing signatures, and leave the recursive
investigation to diffsigs (which relies on ability to list cache
files - not available over http).

The CDN check can be performed by simply running 'bitbake -DD -n'
which is very verbose, but neverthless reports the amount
of missing sstate objects and what they are in a way that can
be programmatically extracted and checked (as suggested by RP).

This also adds local sstate tests, as they can be useful to
determine whether the missing cdn objects were never created or
erroneously cleaned up, or if they were created but didn't propagate
to cdn.

[YOCTO #15303]

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