Fix: tests: metadata presence on relayd is not deterministic

There is no synchronization point guaranteeing the presence of metadata on
lttng-relayd side when the live metadata request is done. It must be
present at some point in the future but might not be at the moment we
perform the request.

Add a retry phase to this step. It must succeed in the future.

Not sure how this test was never marked as flaky.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Change-Id: I033005c6a228013e699c74d8e8faafcb3272dd98
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>