[sint] Refactor test to ensure stanza order
The test that's modified in this commit asserts that upon MUC join, stanzas are received in a particular order.
The previous implementation depended on several event listeners (one for presence, one for messages) that did not always fire in the same order in which the corresponding stanzas arrived. This made the approach unsuitable to reliably test the order in which stanzas arrive.
This commit stops using Smack's MUC API when trying to collect the order in which stanzas arrive. Instead, it joins a chatroom and listens for its stanzas using basic stanza handling. As this uses exactly one stanza listener, that's guaranteed to be invoked in order of stanza arrival, any synchronicity issue in the previous implementation no longer applies.
