test: Fix usage of LoopbackPartitions.

...as well as some other libraries.

TEST=./run_tests
BUG=chromium:954276

Change-Id: I345f9fe5ec2a786ae522853b84368ad1cea633b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1660746
Tested-by: Evan Hernandez <evanhernandez@chromium.org>
Reviewed-by: Alex Klein <saklein@chromium.org>
Auto-Submit: Evan Hernandez <evanhernandez@chromium.org>
diff --git a/api/controller/test_unittest.py b/api/controller/test_unittest.py
index 260d9da..d1b6302 100644
--- a/api/controller/test_unittest.py
+++ b/api/controller/test_unittest.py
@@ -226,7 +226,7 @@
     @contextlib.contextmanager
     def MockLoopbackPartitions(*_args, **_kwargs):
       mount = mock.MagicMock()
-      mount.destination = self.image_mount_dir
+      mount.Mount.return_value = [self.image_mount_dir]
       yield mount
     self.PatchObject(image_lib, 'LoopbackPartitions', MockLoopbackPartitions)