Wait for ClickHouse to report ready before parsing ports (#6655)

This changes the logic for parsing the server ports that ClickHouse
listens on, so that it waits until we are certain that they've been
completely written. This fixes a few flaky tests, currently including at
least #4779, #4972, and #5180. There have been others in the past, which
we've addressed with less-complete solutions that only narrow the race
window. This should eliminate it.