nfts_by_issuer (#948)

Fixes issue #385

Original PR:
#584

unittests/util/MockBackend.h

@@ -107,6 +107,18 @@ struct MockBackend : public BackendInterface {
         (const, override)
     );
 
+    MOCK_METHOD(
+        NFTsAndCursor,
+        fetchNFTsByIssuer,
+        (ripple::AccountID const& issuer,
+         std::optional<std::uint32_t> const& taxon,
+         std::uint32_t const ledgerSequence,
+         std::uint32_t const limit,
+         std::optional<ripple::uint256> const& cursorIn,
+         boost::asio::yield_context yield),
+        (const, override)
+    );
+
     MOCK_METHOD(
         std::vector<Blob>,
         doFetchLedgerObjects,