selftests: net: tsn_lib: allow multiple isochron receivers

Move the PID variable for the isochron receiver into a separate
namespace per stats port, to allow multiple receivers (and/or
orchestration daemons) to be instantiated by the same script.

Preserve the existing behavior by making isochron_do() use the default
stats TCP port of 5000.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Kurt Kanzenbach <kurt@linutronix.de>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>