iotests: Replace assertEquals() with assertEqual()

iotests: Replace assertEquals() with assertEqual()

TestCase.assertEquals() is deprecated since Python 2.7. Recent Python
versions print a warning when the function is called, which makes test
cases fail.

Replace it with the preferred spelling assertEqual().

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>