tests: Portability fixes, replaced 0/1/-1 return codes with macros

tests/string/strcspn.c

@@ -1,10 +1,11 @@
 #include <string.h>
 #include <stdio.h>
+#include <stdlib.h>
 
 int main(void) {
 	char *world = "world";
 	printf("%ld\n", strcspn("hello", world)); // should be 2
 	printf("%ld\n", strcspn("banana", world)); // should be 6
 
-    return 0;
+    return EXIT_SUCCESS;
 }