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

tests/unistd/exec.c

@@ -1,9 +1,10 @@
 #include <unistd.h>
 #include <stdio.h>
+#include <stdlib.h>
 
 int main(void) {
     char* args[] = {"sh", "-c", "echo 'exec works :D'", NULL};
     execv("/bin/sh", args);
     perror("execv");
-    return 0;
+    return EXIT_SUCCESS;
 }