Enhance integration testing setup to enable soft-logout to be triggered by the test cases

Current thoughts:

- a synapse module that hooks in (possibly more than it should) to the homeserver in order to trigger the soft logout process after an API call is made to it
- Modules should not need an update on every synapse release
- but the depth of the change might require us to skirt round the module API and hook directly into the HomeServer instead.
- modules can provide api endpoints for synapse to host, which would be easy to work with.
- the module could also extend to additional things of this nature (what?)

- an ability to root in the database directly to trigger a soft logout flow (without involving synapse)
- possibly has issues with synapse caching layers if they occur in future