tests: Fix Python 2.6 compatibility

Recent tests use set comprehension which is available only in Python
2.7, failing builds on EL machines.

Change-Id: Idf684f2dbee3a024d90d7d9fd7378636fad2ea8d
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31785
Reviewed-by: Dan Kenigsberg <danken@redhat.com>