selftests: forwarding: Make vxlan-bridge-1q pass on debug kernels

selftests: forwarding: Make vxlan-bridge-1q pass on debug kernels

The ageing time used by the test is too short for debug kernels and
results in entries being aged out prematurely [1].

Fix by increasing the ageing time.

[1]
 # ./vxlan_bridge_1q.sh
 [...]
 INFO: learning vlan 10
 TEST: VXLAN: flood before learning                                  [ OK ]
 TEST: VXLAN: show learned FDB entry                                 [ OK ]
 TEST: VXLAN: learned FDB entry                                      [FAIL]
         swp4: Expected to capture 0 packets, got 10.
 RTNETLINK answers: No such file or directory
 TEST: VXLAN: deletion of learned FDB entry                          [ OK ]
 TEST: VXLAN: Ageing of learned FDB entry                            [FAIL]
         swp4: Expected to capture 0 packets, got 10.
 TEST: VXLAN: learning toggling on bridge port                       [ OK ]
 [...]

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/r/20240304095612.462900-5-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>