tests: fix missing `HEALTHCHECK` for tests that check for `healthy` state
tests: fix missing `HEALTHCHECK` for tests that check for `healthy` state

As implemented, the `HEALTHCHECK` will check whether the actual `inspircd`
process is running in the container by check for open port 6667. This check
will run every 1 second (interval), allowing 3 seconds for the actual `CMD` to
return (timeout).  Failures in the first 60 seconds after startup
(start-period) will not count towards an `unhealthy` state. After the
start-period, 3 successive failed checks (retries) will result in an
`unhealthy` state. A successful check at any time after the start of the
container will result in a `healthy` state until 3 successive failed checks.

There was once a similar `HEALTHCHECK` but it was unfortunately lost to the
work in b00ac85a0d94785da3f316082a53118a68419293.

Adam-/inspircd-docker