Mock-like expectations for TestChainSource

Mock-like expectations for TestChainSource

Add a method to TestChainSource to test chain::Filter expectations. This
is limited to register_output, allowing tests to assert that the method
was called with a specific output and dictate what the return value is.

Multiple expectations are checked in the order in which they were added.
Failure occurs if a call doesn't match the next expectation or if there
are unsatisfied expectations. If not expectations are added, then no
calls are checked.