selftests: forwarding: Test changes in mirror-to-gretap

These tests set up mirroring in a situation that the configuration is
incorrect, i.e. mirrored packets, if any, are not supposed to reach
destination tunnel device. Then the configuration is rectified and
mirroring is checked to have started working.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>