testserver: Increase timeout when waiting for healthiness

Most of the test failures we're observing in our CI pipelines right now happen due to Praefect not becoming healthy. This isn't much of a surprise: we need to spawn the Praefect executable, Praefect needs to do its initial setup, connect to the database... and this all needs to finish in at most three seconds, which feels quite optimistic if the machine is highly loaded.

Increase the timeout to 15 seconds. While this may be excessive, I'd rather be safe and have less CI failures.