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).
