Add a query and corresponding result (or error) to the mock backend, meaning
that if it receives a forward search for params through
forward_search (or its asynchronous variants),
the mock backend will return the given results or error to the caller.
Add a query and corresponding result (or error) to the mock backend, meaning
that if it receives a reverse search for params through
reverse_resolve (or its asynchronous variants),
the mock backend will return the given results or error to the caller.