REGTESTS: ssl: Fix the script about automatic SNI selection

REGTESTS: ssl: Fix the script about automatic SNI selection

First, the barrier to delay the client execution was moved before the client
definition. Otherwise, the connection is established too early and with
short timeouts it could be closed before the requests are sent.

The main purpose of the barrier was to workaround slow health-checks. This
is also the reason why the script was flagged as slow. But it can be
significantly speed-up by setting a slow "inter" value. It is now set to
100ms and the script is no longer slow.