selftests: netdevsim: set test timeout to 10 minutes

The longest running netdevsim test, nexthop.sh, currently takes
5 min to finish. Around 260s to be exact, and 310s on a debug kernel.
The default timeout in selftest is 45sec, so we need an explicit
config. Give ourselves some headroom and use 10min.

Commit under Fixes isn't really to "blame" but prior to that
netdevsim tests weren't integrated with kselftest infra
so blaming the tests themselves doesn't seem right, either.

Fixes: 8ff25dac88f6 ("netdevsim: add Makefile for selftests")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>