descriptor: Be able to get the pubkeys involved in a descriptor

src/wallet/test/walletload_tests.cpp

@@ -34,6 +34,7 @@ public:
     std::optional<int64_t> ScriptSize() const override { return {}; }
     std::optional<int64_t> MaxSatisfactionWeight(bool) const override { return {}; }
     std::optional<int64_t> MaxSatisfactionElems() const override { return {}; }
+    void GetPubKeys(std::set<CPubKey>& pubkeys, std::set<CExtPubKey>& ext_pubs) const override {}
 };
 
 BOOST_FIXTURE_TEST_CASE(wallet_load_descriptors, TestingSetup)