tests: More refactoring, add helper header to every test, override exit for better error reporting

tests/string/strchr.c

@@ -1,6 +1,8 @@
 #include <string.h>
 #include <stdio.h>
 
+#include "test_helpers.h"
+
 int main(void) {
 	printf("%s\n", strchr("hello", 'e')); // should be ello
 	printf("%s\n", strchr("world", 'l')); // should be ld