Fix test false positive introduced by threaded I/O.

Now clients that are ready to be terminated asynchronously are processed
more often in beforeSleep() instead of being processed in serverCron().
This means that the test will not be able to catch the moment the client
was terminated, also note that the 'omem' figure now changes in big
steps, because of the new client output buffers layout.

So we have to change the test range in order to accomodate for that.
Yet the test is useful enough to be worth taking, even if its precision
is reduced by this commit. Probably if we get more problems, a thing
that makes sense is just to check that the limit is < 200k. That's more
than enough actually.

tests/unit/obuf-limits.tcl

@@ -15,7 +15,7 @@ start_server {tags {"obuf-limits"}} {
             if {![regexp {omem=([0-9]+)} $c - omem]} break
             if {$omem > 200000} break
         }
-        assert {$omem >= 90000 && $omem < 200000}
+        assert {$omem >= 70000 && $omem < 200000}
         $rd1 close
     }