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

tests/string/strtok.c

@@ -1,6 +1,8 @@
 #include <string.h>
 #include <stdio.h>
 
+#include "test_helpers.h"
+
 int main(void) {
     char source[] = "I'd just like to interject for a moment.  What you're referring to as Linux, "
                     "is in fact, GNU/Linux, or as I've recently taken to calling it, GNU plus Linux.\n";