tests: check marking messages as being replied to

Unfortunately this does not reproduce the issues I'm hitting in that bugreport :(

CCBUG: 329235
Prevent segfault when replying to a message which has disappeared already

The use of a QModelIndex was incorrect; the code was essentially holding a weak
reference to an object which can be deleted in the background. Switching to a
QPersistentModelIndex makes sure that an invalidated index will not be used.

BUG: 329235
CCMAIL: me@jailuthra.in