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

tests/string/strncmp.c

@@ -1,6 +1,8 @@
 #include <string.h>
 #include <stdio.h>
 
+#include "test_helpers.h"
+
 int main(void) {
     printf("%d\n", strncmp("a", "aa", 2));
     printf("%d\n", strncmp("a", "aä", 2));