Tests: add a "new metadata after clear" test

Add a test that validates that the relay daemon performs a
metadata stream rotation after a clear.

The test enables a single event and launches a test application to
trigger it 10 times. Once the 10 event occurrences have been seen
by the live client, the session is cleared.

Then, two new events (statedump start and end) are enabled. This causes
new event descriptions to be appended to the metadata stream.

After a clear, the relay daemon should rotate the metadata stream
and send the new contents to the client. The client will then
be able to decode the statedump start/end events.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ie6057577e3ee8a7c3ed1a84941308d10d372c16c