tests: revived forgotten test

It appears that the original author meant for the writeLargeData to be a
test, but forgot the `test' prefix.
Moreover, previously the test asserted data>4096, where data is a
string, which in python is always True. I simplified it a bit, and
removed the trivial assertion.

Change-Id: I1a847e83046ea12596076fabaa42baba6939e1cd
Signed-off-by: Yoav Kleinberger <ykleinbe@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/29780
Reviewed-by: Vered Volansky <vvolansk@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>