selftest/reproducible: Add ability to pull some objects from sstate

When debugging why a single recipe doesn't reproduce, its a pain
to wait for the world to rebuild from scratch. Update the selftest
to allow this to be configured, for example you could set
targets as ['perf'] and sstate_targets as ['virtual/kernel']
and then it should only be rebuilding perf in the test rather than
things like the toolchain (parts of the kernel may be unavoiable
as they're not in sstate).

Can be run as:

OEQA_DEBUGGING_SAVED_OUTPUT=/tmp/perf-diffoscope oe-selftest -r reproducible.ReproducibleTests.test_reproducible_builds

to save diffoscope output.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>