selftests/net/forwarding: add slowwait functions

Add slowwait functions to wait for some operations that may need a long time
to finish. The busywait executes the cmd too fast, which is kind of wasting
cpu in this scenario. At the same time, if shell debugging is enabled with
`set -x`. the busywait will output too much logs.

Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://lore.kernel.org/r/20240205130048.282087-2-liuhangbin@gmail.com


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