selftests: forwarding: Fix ping failure due to short timeout

The tests send 100 pings in 0.1 second intervals and force a timeout of
11 seconds, which is borderline (especially on debug kernels), resulting
in random failures in netdev CI [1].

Fix by increasing the timeout to 20 seconds. It should not prolong the
test unless something is wrong, in which case the test will rightfully
fail.

[1]
 # selftests: net/forwarding: vxlan_bridge_1d_port_8472_ipv6.sh
 # INFO: Running tests with UDP port 8472
 # TEST: ping: local->local                                            [ OK ]
 # TEST: ping: local->remote 1                                         [FAIL]
 # Ping failed
 [...]

Fixes: b07e9957 ("selftests: forwarding: Add VxLAN tests with a VLAN-unaware bridge for IPv6")
Fixes: 728b3525

 ("selftests: forwarding: Add VxLAN tests with a VLAN-aware bridge for IPv6")
Reported-by: Paolo Abeni <pabeni@redhat.com>
Closes: https://lore.kernel.org/netdev/24a7051fdcd1f156c3704bca39e4b3c41dfc7c4b.camel@redhat.com/


Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20240320065717.4145325-1-idosch@nvidia.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>