selftests/net/lib: update busywait timeout value

The busywait timeout value is a millisecond, not a second. So the
current setting 2 is too small. On slow/busy host (or VMs) the
current timeout can expire even on "correct" execution, causing random
failures. Let's copy the WAIT_TIMEOUT from forwarding/lib.sh and set
BUSYWAIT_TIMEOUT here.

Fixes: 25ae948b

 ("selftests/net: add lib.sh")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240124061344.1864484-1-liuhangbin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>