* test/engine/api/geary-email-properties-test.vala: Add new mock
  EmailProperties object.

* test/engine/app/app-conversation-test.vala (ConversationTest): Set
  email sent and received dates to suppress warnings adding them to
  Conversation instances.

* test/engine/app/app-conversation-set-test.vala (ConversationSetTest):
  Set email sent and received dates to suppress warnings adding them to
  Conversation instances, handle both casees when merging two
  conversations, that the first was merged to the second and vice versa.