selftests/net: Add missing config options for PMTU tests

PMTU tests in pmtu.sh need support for VTI, VTI6 and dummy
interfaces: add them to config file.

Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Fixes: d1f1b9cb ("selftests: net: Introduce first PMTU test")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>