Fix probe test

Fix for a probe test that failed every once in a
while due to the early-majority change previously
committed. Sometimes a write would return success
before the third node had succeeded and the probe
test would look for on-disk evidence and fail,
when it would've been fine had it waited just a
bit longer for the third node to complete.

Since there's no real way for the probe test to
know when all three nodes are done, I just made
it retry once a second for several seconds before
reporting an error.

There may be more tests like this we'll have to
fix as we run across them.

Change-Id: I749e43d4580a7c726a9a8648f71bafefa70a05f5
Merge "Fix probe test"