REGTESTS: proxy: test monitor-uri and the monitor fail condition

REGTESTS: proxy: test monitor-uri and the monitor fail condition

"monitor fail" had no coverage and "monitor-uri" was only used as a
side setting of a health check test.

The test checks that the monitor URI is answered by the frontend
without reaching the backend while the other URIs are still forwarded,
that the status switches from 200 to 503 when the condition is met,
that the "unless" form inverts that, and that the monitor response
goes through the error files of the proxy as documented.