hdkeychain: address go vet warnings

hdkeychain/bench_test.go

@@ -79,6 +79,6 @@ func BenchmarkSerialize(b *testing.B) {
 	b.StartTimer()
 
 	for i := 0; i < b.N; i++ {
-		masterKey.String()
+		_ = masterKey.String()
 	}
 }

hdkeychain/extendedkey_test.go

@@ -232,7 +232,8 @@ tests:
 		}
 
 		if extKey.Depth() != uint8(len(test.path)) {
-			t.Errorf("Depth of key %d should match fixture path")
+			t.Errorf("Depth of key %d should match fixture path: %v",
+				extKey.Depth(), len(test.path))
 			continue
 		}