import mock from third_party explicitly

Stop relying on third_party/ leaking into sys.path all the time.

BUG=b:187794934
TEST=CQ passes

Change-Id: If233859063e6d2a1f2fdaa851febcbe8f536896a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2896053
Tested-by: Mike Frysinger <vapier@chromium.org>
Auto-Submit: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Chris McDonald <cjmcdonald@chromium.org>
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
diff --git a/api/controller/firmware_unittest.py b/api/controller/firmware_unittest.py
index 9b24493..a14bb09 100644
--- a/api/controller/firmware_unittest.py
+++ b/api/controller/firmware_unittest.py
@@ -8,8 +8,6 @@
 
 import os
 
-import mock
-
 from google.protobuf import json_format
 
 from chromite.api import api_config
@@ -18,6 +16,7 @@
 from chromite.lib import constants
 from chromite.lib import cros_build_lib
 from chromite.lib import cros_test_lib
+from chromite.third_party import mock
 
 
 class BuildAllFirmwareTestCase(cros_test_lib.MockTempDirTestCase,