Swtich several tests to GSContextMock.

Mocking out individual gs context methods leads to issues in
run_tests, so switch to using the full GSContextMock in
validation_pool_unittest.py, cache_unittest.py, and pushimage_unittest.py.

BUG=chromium:482255
TEST=Ran tests inside/outside run_tests with/without empty .cache.

Change-Id: If17c2d45aa4f2690331f5d819d8cced69fd918e6
Reviewed-on: https://chromium-review.googlesource.com/268395
Commit-Queue: Don Garrett <dgarrett@chromium.org>
Tested-by: Don Garrett <dgarrett@chromium.org>
Reviewed-by: David James <davidjames@chromium.org>
diff --git a/scripts/pushimage_unittest.py b/scripts/pushimage_unittest.py
index d4031c2..76618e4 100644
--- a/scripts/pushimage_unittest.py
+++ b/scripts/pushimage_unittest.py
@@ -22,13 +22,15 @@
 class InputInsnsTest(cros_test_lib.MockTestCase):
   """Tests for InputInsns"""
 
+  def setUp(self):
+    self.StartPatcher(gs_unittest.GSContextMock())
+
   def testBasic(self):
     """Simple smoke test"""
-    with mock.patch.object(gs.GSContext, 'Exists', return_value=False):
-      insns = pushimage.InputInsns('test.board')
-      insns.GetInsnFile('recovery')
-      self.assertEqual(insns.GetChannels(), ['dev', 'canary'])
-      self.assertEqual(insns.GetKeysets(), ['stumpy-mp-v3'])
+    insns = pushimage.InputInsns('test.board')
+    insns.GetInsnFile('recovery')
+    self.assertEqual(insns.GetChannels(), ['dev', 'canary'])
+    self.assertEqual(insns.GetKeysets(), ['stumpy-mp-v3'])
 
   def testGetInsnFile(self):
     """Verify various inputs result in right insns path"""