Add test for ensuring keys are available locally prior to backup reset

[kegsay]

See element-hq/element-android#8814

Test via:

• Alice logs in, sends messages in E2EE rooms to make some keys.
• Alice logs in on a new device. We expect the client to download room keys.
• Alice on the new device resets key backup. The client must have downloaded all room keys in the backup by this point, prior to resetting.
• Alice on the new device opens up the E2EE rooms and checks she can decrypt the messages sent.