cli: device_imager_unittest: Fixup fd leak

I didn't apply the appropriate side_effect parameter uniformly in:
https://chromium-review.googlesource.com/c/chromiumos/chromite/+/4726921

BUG=none
TEST=./run_tests

Change-Id: I8d5587cd05e2257df1867d9031eed9c2969d42c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/4772016
Tested-by: Brian Norris <briannorris@chromium.org>
Reviewed-by: Jae Hoon Kim <kimjae@chromium.org>
Auto-Submit: Brian Norris <briannorris@chromium.org>
Commit-Queue: Brian Norris <briannorris@chromium.org>
diff --git a/cli/device_imager_unittest.py b/cli/device_imager_unittest.py
index 28bdb30..397ce40 100644
--- a/cli/device_imager_unittest.py
+++ b/cli/device_imager_unittest.py
@@ -698,7 +698,7 @@
     """Tests stateful payload generator."""
 
     @mock.patch.object(paygen_stateful_payload_lib, "GenerateStatefulPayload")
-    @mock.patch.object(os, "close")
+    @mock.patch.object(os, "close", side_effect=os.close)
     def testRun(self, os_close_mock, paygen_mock):
         """Tests run() function."""
         image = "/foo/image"