tests: Make pyflakes happy with apparently unused variable

Pyflakes was upset by what looks like an unused local variable.  The
local variable was used to keep a reference to a Receiver instance that
must be live during this test.

Change-Id: I47dccda40a6ce84e6ff8cac03eeb22ae2b7b5804
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/36484
Reviewed-by: Dan Kenigsberg <danken@redhat.com>