lib: unit tests: improve / better coverage

lib/mu-config.cc

@@ -79,6 +79,16 @@ test_basic()
 	}
 }
 
+static void
+test_read_only()
+{
+	MemDb db{true/*read-only*/};
+	Config conf_db{db};
+
+	auto res = conf_db.set<Config::Id::MaxMessageSize>(12345);
+	g_assert_false(!!res);
+}
+
 int
 main(int argc, char* argv[])
 {
@@ -86,6 +96,7 @@ main(int argc, char* argv[])
 
 	g_test_add_func("/config-db/props", test_props);
 	g_test_add_func("/config-db/basic", test_basic);
+	g_test_add_func("/config-db/read-only", test_read_only);
 
 	return g_test_run();
 }