qmc-example: refactor QMCTest to properly order actions
qmc-example: refactor QMCTest to properly order actions

To be more specific:
- No race condition in running tests when the test room is already
joined; joining occurs before the very first sync.
- qmc-example doesn't (in vain) wait for the last sync in order to make
sure the final message ("All tests finished") is delivered - uses
Room::messageSent() instead now.
- Running QMCTest::loadMembers() does not rely on having a test room

Fxrh/libqmatrixclient,QMatrixClient/libqmatrixclient,QMatrixClient/libqmatrixclient