wcfs: tests: Don't forget to close ._wcfuseabort fd

The function to do it was there, but I missed to add corresponding defer.
Fixes 6f0cdaff (wcfs: Provide isolation to clients).

wcfs/wcfs_test.py

@@ -374,6 +374,7 @@ class tWCFS(_tWCFS):
     def close(t):
         def _():
             os.close(t._wcfuseabort)
+        defer(_)
         defer(t._closed.close)
 
         # unmount and wait for wcfs to exit