Remove MockClock() (#18992)

Spawning from adding some logcontext debug logs in
https://github.com/element-hq/synapse/pull/18966 and since we're not
logging at the `set_current_context(...)` level (see reasoning there),
this removes some usage of `set_current_context(...)`.

Specifically, `MockClock.call_later(...)` doesn't handle logcontexts
correctly. It uses the calling logcontext as the callback context
(wrong, as the logcontext could finish before the callback finishes) and
it didn't reset back to the sentinel context before handing back to the
reactor. It was like this since it was [introduced 10+ years
ago](38da9884e7).
Instead of fixing the implementation which would just be a copy of our
normal `Clock`, we can just remove `MockClock`

tests/test_state.py

@@ -29,7 +29,6 @@ from typing import (
     Optional,
     Set,
     Tuple,
-    cast,
 )
 from unittest.mock import AsyncMock, Mock
 
@@ -43,12 +42,11 @@ from synapse.events.snapshot import EventContext
 from synapse.state import StateHandler, StateResolutionHandler, _make_state_cache_entry
 from synapse.types import MutableStateMap, StateMap
 from synapse.types.state import StateFilter
-from synapse.util.clock import Clock
 from synapse.util.macaroons import MacaroonGenerator
 
 from tests import unittest
-
-from .utils import MockClock, default_config
+from tests.server import get_clock
+from tests.utils import default_config
 
 _next_event_id = 1000
 
@@ -248,7 +246,7 @@ class StateTestCase(unittest.TestCase):
                 "hostname",
             ]
         )
-        clock = cast(Clock, MockClock())
+        reactor, clock = get_clock()
         hs.config = default_config("tesths", True)
         hs.get_datastores.return_value = Mock(
             main=self.dummy_store,