Fix test/utils.py Twisted log formatting hack

[matrixbot]

This issue was originally created by @ShadowJonathan at matrix-org/sydent#315.

Apply the same fix from matrix-org/synapse#8335 onto the following lines:

https://github.com/matrix-org/sydent/blob/66abd5b6808cd9e15e62e88000ec7e5afc287d56/tests/utils.py#L234-L245

[matrixbot]

This comment was originally posted by @anoadragon453 at matrix-org/sydent#315 (comment).

@ShadowJonathan would you mind creating a similar PR for this as the one you did for Synapse?