Chromad: Fix kerberos files tests

Modify KerberosFilesChangeWaiter to be more robust callbacks called
several times

BUG=chromium:865206

Change-Id: I0c5f22fb26871607257d3b29ad3bd88d5b505c7c
Reviewed-on: https://chromium-review.googlesource.com/1145074
Commit-Queue: Alexander Alekseev <alemate@chromium.org>
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576941}
1 file changed