[#5708] ticket:262 Added test that reproduces the bug
[#5708] ticket:262 Don't move tracker's Discussion instance with a ticket

All tickets that belongs to the same tracker shares one Discussion instance.
We should re-attach discussion thread to Discussion instance of a destination tracker,
and shouldn't touch discussion of a source tracker.
[#5708] ticket:262 Fixed failing test due to discussion handling when moving the ticket
[#5691] ticket:263 Test for incorect ticket references in notifications