selftests: forwarding: Make VXLAN ECN encap tests more robust

These tests sometimes fail on the netdev CI because the expected number
of packets is larger than expected [1].

Make the tests more robust by specifically matching on VXLAN
encapsulated packets and allowing up to five stray packets instead of
just two.

[1]
 [...]
 # TEST: VXLAN: ECN encap: 0x00->0x00                                  [FAIL]
 # v1: Expected to capture 10 packets, got 13.
 # TEST: VXLAN: ECN encap: 0x01->0x01                                  [ OK ]
 # TEST: VXLAN: ECN encap: 0x02->0x02                                  [ OK ]
 # TEST: VXLAN: ECN encap: 0x03->0x02                                  [ OK ]
 [...]

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/r/20240304095612.462900-6-idosch@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>