We've changed the contract for PlanResourceChange to now require the
provider to populate any default values (including unknowns) it wants to
set for computed arguments, so our mock provider here now needs to be a
little more complex to deal with that.
This fixes several of the tests in this package. A minor change to
TestLocal_applyEmptyDirDestroy was required to make it properly configure
the mock provider so PlanResourceChange can access the schema.
e3618f915b