selftests: net: local_termination: parameterize sending interface

In future changes we will want to subject the DUT, $h2, to additional
VLAN-tagged traffic. For that, we need to run the tests using $h1.100 as
a sending interface, rather than the currently hardcoded $h1.

Add a parameter to run_test() and modify its 2 callers to explicitly
pass $h1, as was implicit before.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>