Add simple /purge_room admin API test

[erikjohnston]

Depends on matrix-org/synapse#6307

[richvdh]

hrm. I think we've previously decided that testing the synapse-specific endpoints in sytest was a mistake (since it makes it harder to apply sytest to other server impls)

honestly I'd be happy to rely on the UT in matrix-org/synapse#6307 and just bin this

[erikjohnston]

That's probably fair, though we probably want to think about what to do about the other tests in the file

[erikjohnston]

I guess one thing that this does test is if they work in worker mode?

[richvdh]

still kinda think we want to test that in synapse/tests rather than here... even if that means doing a bunch of annoying boilerplatey work to get a pair of workers set up in a trial env.

[richvdh]

what to do about the other tests in the file

kill them

kill them all

MWAHAHAHAHAHA