Tests: avoid edge cases in upstream random two test.

Unavailable servers contribute to the number of attempts, if selected,
before the balancer would fall back to the default round-robin method.
This means that it's quite possible to get server with more connections.
To facilitate with selecting two alive servers, down server was removed
from the upstream configuration at the cost of slightly worse coverage.
diff --git a/upstream_random.t b/upstream_random.t
index 0b72fa9..73713d8 100644
--- a/upstream_random.t
+++ b/upstream_random.t
@@ -49,7 +49,6 @@
         random two;
-        server down;
     upstream w {