e2e: make it deterministic

Sometimes the tests fail because of a race condition, some messages are sent in an arbitrary order (for example when a user joins a room with an occupant, his/her self presence may be sent to all the occupants or to him/herself, depending on if the IRC server’s response was received fast or slowly).

We need to be able to regroup sequenced “expect_stanza”, and the the following received stanza must match one of this group, until they are all received.

This will be useful to be able to run the e2e tests through valgrind, because it makes the process really slower and creates a lot of these issues.