lib-http: test-http-client-errors - Allow more relaxed timeouts for connect retry...

lib-http: test-http-client-errors - Allow more relaxed timeouts for connect retry tests

With max_connect_attempts=3 the connects come at (0ms, 100ms, 300ms).
Before the 3rd attempt a timeout at 250ms must have triggered, so there
was only 50ms time for it to trigger. This wasn't always enough when
running with valgrind on an overloaded system. Solve this by increasing
max_connect_attempts=4 so the 4th attempt comes at 700ms, giving the
timeout 450ms to trigger.