zcash_client_backend: Reduce the dependency of the metadata_queries_exclude_unwanted_notes test on zcash_client_sqlite behavior.

[nuttycom]

An approach to fixing this would be to construct an input source that only selects the note(s) with a given nullifier. Then the caller could specify "I only want to spend these specific notes", and those are all that we'd get back from note selection.

Originally posted by @str4d in #1580 (comment)