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

tests/string/mem.c

@@ -2,6 +2,8 @@
 #include <stdlib.h>
 #include <string.h>
 
+#include "test_helpers.h"
+
 int main(void) {
     puts("# mem #");
     char arr[100];