zcash_client_sqlite: Create a test that exercises a Proposal with transparent inputs

[str4d]

WalletDb::get_unspent_transparent_output is only used in proto::proposal::Proposal::try_into_standard_proposal. The latter is exercised in the tests, but only with shielded inputs. We should add a shielding test to ensure this pathway is covered (and enable finding bugs like #1326).