tests,aix: avoid running fsync on directory

The underlying fsync system call returns EBADF on a file descriptor
for an open directory. So avoid running fsync on it.

test/parallel/test-fs-utimes.js

@@ -35,7 +35,11 @@ function stat_resource(resource) {
   if (typeof resource === 'string') {
     return fs.statSync(resource);
   } else {
+    const stats = fs.fstatSync(resource);
     // ensure mtime has been written to disk
+    // except for directories on AIX where it cannot be synced
+    if (common.isAIX && stats.isDirectory())
+      return stats;
     fs.fsyncSync(resource);
     return fs.fstatSync(resource);
   }