gerrit: no IRC message for test/gerrit-ping

Due to bug 46917, we are sending a ping every five minutes on the
test/gerrit-ping.git repository.  We do not want IRC notifications to be
sent though, this patch sends the notifications to 'ignored.log' which
is not read by ircecho.

Change-Id: I431ff970596aae03b3ab5276303ed18574892b13