selftests: forwarding: mirror_gre_changes: Fix TTL test

When running in a simulator, the TTL change takes a while to settle and
during this time the performance of the packet processing is lowered.
The resulting instability leads to ping sending more packets as it
assumes some have been dropped. This then leads to regular spurious
failures as more packets than expected are observed.

Sleep a bit to give the system time to stabilize.

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