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

tests/unistd/fsync.c

@@ -1,6 +1,7 @@
 #include <unistd.h>
 #include <fcntl.h>
 #include <stdio.h>
+#include <stdlib.h>
 
 int main(void) {
     int fd = open(".", 0, 0);
@@ -8,5 +9,5 @@ int main(void) {
     status = fsync(fd);
     printf("fsync exited with status code %d\n", status);
     close(fd);
-    return 0;
+    return EXIT_SUCCESS;
 }