Use MagicMock as return value for _get_diff mock

The `_get_diff` method of TestSyncer class is mocked using an AsyncMock object. By default, when an AsyncMock object is called **and awaited**, it returns a child mock of the same time (another AsyncMock) according to the "the child is a like the parent" principle. This means that the _get_diff method will return an AsyncMock unless a different return_value is explicitly provided.

Because of that "child is like parent" behavior, this will happen in lines 194-196 of bot.cogs.sync.syncers (annotations added by me):

```
// `diff` will be a child AsyncMock as "child is like parent"
diff = await self._get_diff(guild)

// `diff._asdict` will be an AsyncMock as "child is like parent" and,
// after being called, it will return an unawaited coroutine object
// we assign the name `diff_dict`:
diff_dict = diff._asdict()

// `diff_dict` is still an unawaited coroutine object meaning that it
// doesn't have an `items()` method:
totals = {k: len(v) for k, v in diff_dict.items() if v is not None}
```

Original, unannotated: https://github.com/python-discord/bot/blob/c81a4d401ea434e98b0a1ece51d3d10f1a3ad226/bot/cogs/sync/syncers.py#L194-L196

This will lead to the following exception when running the tests:

```py
======================================================================
ERROR: test_sync_confirmation_context_redirect (tests.bot.cogs.sync.test_base.SyncerSyncTests) (ctx=None, author=<MockMember name='mock.user' spec_set='Member' id='140583452034864'>, message=None)
If ctx is given, a new message should be sent and author should be ctx's author.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/sebastiaan/pydis/repositories/bot/tests/bot/cogs/sync/test_base.py", line 348, in test_sync_confirmation_context_redirect
    await self.syncer.sync(guild, ctx)
  File "/home/sebastiaan/pydis/repositories/bot/bot/cogs/sync/syncers.py", line 196, in sync
    totals = {k: len(v) for k, v in diff_dict.items() if v is not None}
AttributeError: 'coroutine' object has no attribute 'items'
```

The solution is to assign an explicit return value so the parent mock doesn't "guess" and return an object of its own type. I previously did that by providing a specific `_Diff` object as the return value, but I should have gone with a `MagicMock` to signify that it's not an important return value; it's just something that needs to support/mimic the API we use on it. So that's what this commit adds.

1 files changed, 2 insertions, 2 deletions

diff --git a/tests/bot/cogs/sync/test_base.py b/tests/bot/cogs/sync/test_base.py
index 17aa4198b..d17a27409 100644
--- a/tests/bot/cogs/sync/test_base.py
+++ b/tests/bot/cogs/sync/test_base.py
@@ -339,8 +339,8 @@ class SyncerSyncTests(unittest.IsolatedAsyncioTestCase):
                 if ctx is not None:
                     ctx.send.return_value = message
 
-                diff = _Diff({1, 2, 3}, {4, 5}, None)
-                self.syncer._get_diff.return_value = diff
+                # Make sure `_get_diff` returns a MagicMock, not an AsyncMock
+                self.syncer._get_diff.return_value = mock.MagicMock()
 
                 self.syncer._get_confirmation_result = mock.AsyncMock(return_value=(False, None))