Remove unneeded Facts() init.

Remove mocking Facts in test_registration, no longer
needed with fixture injection.
Store default inject stub Facts on SubManFixture
test_async does not need to mock Facts

Use injected version here, passed to PoolStash
until PoolStash stops passing in facts.
SubManFixture's mock identity now NonCallable

Fix cases (facts_gui display_system_id) that try to
check identity.uuid or identity.name for None'ness

Normal cases should call exists_and_valid(), but
factsgui tries to avoid passing None to gtk.
factsgui identity now injected at the last minute

Identity always exists before creating SystemFactsDialog,
so don't require it till we need it, to avoid a stale
self.identity

test_facts_gui doesn't need to create a Identity,
SubManFixture does that already.