fw: optimize FwNccStrategy/FavorRespondingUpstream test case

This test case previously relies on precise timing, and the slack is 3ms
in the worst case. Failure rate is high if system is slow.
This commit optimizes this tests case with several operation-limited
io runs, so that it doesn't rely on precise timing.

refs #2037

Change-Id: I80b87d830dcf8d87fce511e79eb9349f75c1c34d