Twisted trunk makes a change to the `TLSMemoryBIOFactory` where
the underlying protocol is changed from `TLSMemoryBIOProtocol` to
`BufferingTLSTransport` to improve performance of TLS code (see
https://github.com/twisted/twisted/issues/11989).
In order to properly hook this code up in tests we need to pass the test
reactor's clock into `TLSMemoryBIOFactory` to avoid the global (trial)
reactor being used by default.
Twisted does something similar internally for tests:
157cd8e659/src/twisted/web/test/test_agent.py (L871-L874)
