Add integration test for timestamp behavior

[KonradBreitsprecherBkd]

The timestamp behavior had no integration test. The test covers:

Verify timestamp behavior from the synchronized participant perspective:

• Sync sender: outgoing event carries virtual send time.
• Async sender: outgoing events will have an invalid timestamp
• Sync receiver: incoming event from async or sync sender is timestamped with sync receive time.
• Async receiver: incoming event from async or sync sender keeps the timestamp (async sender: invalid timestamp, sync sender: send time)