unix: Test the --external instead of --ext-unix-sk

The latter option should hide, the official API is the --external.
This patch tests the option, thus completing the deprecation.

The legacy -x|--ext-unix-sk test is still in zdtm/static/socket-ext.

travis-ci: success for unix: Test the --external instead of --ext-unix-sk
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>