tests: do not assume that the image description is immediately ready

This patch is for our CM&HDR protocol extension test.

According to the protocol, the compositor may take the time it needs
before sending 'ready' or 'failed' for a certain image description that
the client creates through the CM&HDR protocol extension.

In our CM&HDR tests, we are assuming that the image description would
be ready immediately. Do not assume that. Instead, let's wait until
the compositor sends one of the events ('failed' or 'ready').

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>