add default behavior to the MockProvider

Add reasonable default behavior to the mock provider, so that may do not
need to depend on the idiosyncrasies of the old (though updated) test
testDiffFn and to a lesser extend the testApplyFn. This behavior is
based directly upon the documented resource lifecycle, rather
than be an ad-hoc collection of behaviors from old tests.

As a proof of concept, remove all uses of testDiffFn from the plan
context tests that don't cause the tests to fail.