Fix SAS verification test
There's 3 things going on in this commit:
1. `this` is maintained in the tests. Some binds are added instead of the `.call(this, ...)` syntax.
2. We use the right `origSendToDevice`
3. We ensure `downloadKeys` is actually on the client
The combination of these 3 fixes makes the test pass.
Part of https://github.com/vector-im/riot-web/issues/11520
