tests: fix ut_conversation

+ testSetMessageDisplayed and testSetMessageDisplayedPreference were
broken because the lastDisplayed behaviour was recently changed to
support syncing across devices. Update the two related tests. Also,
avoid to update the lastDisplayed on merge
+ testSyncWithoutPinnedCert was badly written causing some sporadic
failures.

Change-Id: I364818b4ececb0fa63e87441f55a7da76fe1feb6