Allow UI introspection of test environments

[kegsay]

If you make a room with Alice and Bob and send messages, it would be nice if we could provide a way for Element X or Element Web R to hook into one of these accounts so the test author can:

• see if the test configuration makes sense
• sanity check for any obvious errors that the test is not catching

For Element X this would just involve dumping the sqlite directory somewhere and then making the app read it. For Element Web R this would involve exposing the Chrome session of the client instance. The latter feels doable at this stage.

[kegsay]

..except it need an entire element-web to be hosted, and that may not be storing the right things in the right places in localstorage/indexeddb. Maybe the EX route is easier then..?