REGTESTS: checks: test the health adjusting from observed traffic

REGTESTS: checks: test the health adjusting from observed traffic

The "observe", "error-limit" and "on-error" server settings had no
coverage. They adjust the health of a server from the traffic it serves
rather than from the health checks, which makes them testable without
any timer: the state changes exactly when the configured number of
consecutive errors has been served.

The test checks that a 404 is not counted as a layer 7 error while a
500 is, that the server is marked down once the error limit is reached
and that the backend then answers 503, and that a failed connection is
enough in layer 4 mode. It also verifies that the resulting state
change is attributed to the health analysis and not to a check result.

The health checks are directed at a second haproxy instance which
accepts the connections and never answers, with "option httpchk" so
that no check can complete and interfere with the observation.