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

tests/stdlib/mktemp.c

@@ -2,6 +2,8 @@
 #include <stdio.h>
 #include <string.h>
 
+#include "test_helpers.h"
+
 int main(void) {
     char* string = (char*) calloc(20, sizeof(char));
     strcpy(string, "tempXXXXXX");