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

tests/string/strchr.c

@@ -1,10 +1,11 @@
 #include <string.h>
 #include <stdio.h>
+#include <stdlib.h>
 
 int main(void) {
 	printf("%s\n", strchr("hello", 'e')); // should be ello
 	printf("%s\n", strchr("world", 'l')); // should be ld
 	printf("%i\n", strchr("world", 0) == NULL); // should be 1
 
-    return 0;
+    return EXIT_SUCCESS;
 }