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

tests/stdio/all.c

@@ -1,6 +1,8 @@
 #include <stdio.h>
 #include <stdlib.h>
 
+#include "test_helpers.h"
+
 int main(void) {
 	FILE *f = fopen("stdio/stdio.in", "r");
 	printf("%c\n", fgetc(f));