selftests: net: increase inter-packet timeout in udpgro.sh

The mentioned test is not very stable when running on top of
debug kernel build. Increase the inter-packet timeout to allow
more slack in such environments.

Fixes: 3327a9c4 ("selftests: add functionals test for UDP GRO")
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/b0370c06ddb3235debf642c17de0284b2cd3c652.1752163107.git.pabeni@redhat.com


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