erasure-code: unfound test race condition

If osd.1 is not instructed to misbehave before the existing_%d objects
are added, they may have chunks on all four OSDs and therefore be
readable even after osd.1 is lost because it has chunks in three other osds.

http://tracker.ceph.com/issues/9387 Fixes: #9387

Signed-off-by: Loic Dachary <loic-201408@dachary.org>