descriptors: Change Parse to return vector of descriptors

When given a descriptor which contins a multipath derivation specifier,
a vector of descriptors will be returned.

src/qt/test/wallettests.cpp

@@ -218,8 +218,10 @@ std::shared_ptr<CWallet> SetupDescriptorsWallet(interfaces::Node& node, TestChai
     // Add the coinbase key
     FlatSigningProvider provider;
     std::string error;
-    std::unique_ptr<Descriptor> desc = Parse("combo(" + EncodeSecret(test.coinbaseKey) + ")", provider, error, /* require_checksum=*/ false);
-    assert(desc);
+    auto descs = Parse("combo(" + EncodeSecret(test.coinbaseKey) + ")", provider, error, /* require_checksum=*/ false);
+    assert(!descs.empty());
+    assert(descs.size() == 1);
+    auto& desc = descs.at(0);
     WalletDescriptor w_desc(std::move(desc), 0, 0, 1, 1);
     if (!wallet->AddWalletDescriptor(w_desc, provider, "", false)) assert(false);
     CTxDestination dest = GetDestinationForKey(test.coinbaseKey.GetPubKey(), wallet->m_default_address_type);