Make two WhatsApp tests independent of the load

The test of a private batch gave its two messages with 50 ms of real
quiet time between them, so on a loaded machine the first message closed
its batch alone and the test failed. The messages now come during a
catch-up, which holds the batch open until its end, so the test no
longer depends on the speed of the machine.

The test of a long disconnection waits 300 ms before the notice and 400
ms for a notice that must not come, so that a slow machine does not give
a notice between two events of the test.

Issue: 6f69ca1