erasure-code: unfound test needs a non empty file

Other rados put will fail as follows

$ touch /tmp/bar
$ ./rados -p rbd put existing_3 /tmp/bar
$ ./rados -p rbd put existing_3 /tmp/bar
WARNING: could not create object: existing_3
error putting rbd/existing_3: (17) File exists

it should be considered a bug in the rados command line but needs to be
addressed separately.

http://tracker.ceph.com/issues/9387 Fixes: #9387

Signed-off-by: Loic Dachary loic-201408@dachary.org