tests: Move securable tests to its own test module

The tests were hidden in  tests/main.py making it harder to find the
tests.

Change-Id: I40fad51de67ad732d9a91119010a6bf889af6bc7
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/24482
Reviewed-by: Dan Kenigsberg <danken@redhat.com>