Add e2e tests that use fully timestamp massaged rooms

[MadLittleMods]

Currently, we can only timestamp massage the events sent in the room after creation using /send?ts=123 and /state?ts=123 but this leaves the primordial room creation and federated join/membership events still at current time which throws things off sometimes.

I've created a few MSC's to track getting us to fully timestamp massaged rooms that appear to be created back in time:

• MSC3997: Add timestamp massaging to /createRoom?ts=123
• MSC3998: Add timestamp massaging to /join?ts=123 and /knock?ts=123

Also tracked in Synapse by matrix-org/synapse#15346

But it's also good to test the scenario with our current flawed setup. This is basically what the Gitter historical rooms look like where the room creation happened at time A and the events look like they were sent before time A.

Todo

• Create/wait for unstable versions in Synapse to use
• Add test variations for fully timestamp massaged rooms
• Keep test variation around for partial timestamp massaged rooms as we have now (useful to make sure Gitter-like historical rooms work)
• Add more tests for timestamp loop situations in the timeline