Use _make_test_img whenever possible.  This way, we will not ignore
user-specified image options.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-id: 20191107163708.833192-15-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>