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

tests/string/strncmp.c

@@ -1,5 +1,6 @@
 #include <string.h>
 #include <stdio.h>
+#include <stdlib.h>
 
 int main(void) {
     printf("%d\n", strncmp("a", "aa", 2));
@@ -9,5 +10,5 @@ int main(void) {
     printf("%d\n", strncmp("a", "c", 1));
     printf("%d\n", strncmp("a", "a", 2));
 
-    return 0;
+    return EXIT_SUCCESS;
 }