Unignore and enforce check E731 ("Do not assign a lambda expression, use a def")

This issue has been migrated from [#9369](https://github.com/matrix-org/synapse/issues/9369).

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(`E731` checks "Do not assign a lambda expression, use a def")

This check is voilated in only a few cases;
```
synapse/util/caches/descriptors.py:500:5: E731 do not assign a lambda expression, use a def
synapse/util/caches/descriptors.py:542:5: E731 do not assign a lambda expression, use a def
tests/server.py:332:9: E731 do not assign a lambda expression, use a def
```
The first to related to caching and then wrapping the callback functions in lambdas, and the last one being `ThreadedMemoryReactorClock.callFromThread`, I think it's better to remove this violation from the ignore list, and then to add `# noqa: E731` statements on those specific places.