fix: remove contradictory timing in health handler test

[umputun]

TestHttp_healthHandler has two pieces of timing that contradict each other. The server gets a 500ms context at line 67, and line 110 then waits up to a full second for that same server to answer. On a machine slow enough to take more than about 450ms to bind and serve, the context cancels the server while require.Eventually is still retrying, and the test reports "server failed to start" having actually started it. Separately, a fixed 50ms sleep stood in for discovery loading five static rules at a 10ms interval; when that is short the health check sees fewer than four services and the count assertion fails instead.

The server now gets a context that outlives the assertions, and the sleep is replaced by waiting on svc.Mappers() reaching five.

I could not reproduce the original failure red on my machine, and neither could a 40-run pass at -cpu=1 under load. Forcing the server context to expire during the wait does produce the exact observed failure, which is what the first change removes:

--- FAIL: TestHttp_healthHandler (1.05s)
        	Messages:   	server failed to start

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