Fix some compilation warnings.

Redone the commit based on the input in
https://github.com/godotengine/godot/pull/7851 . Not all warnings were
fixed but it's a start.

main/tests/test_string.cpp

@@ -835,7 +835,6 @@ bool test_28() {
 
 bool test_29() {
 
-	bool error = false;
 	bool state = true;
 	bool success = false;