selftests: forwarding: Move lldpad waiting to lib.sh

The function lldpad_wait() will be useful for a test added by a
following patch. Likewise would the "sleep 5" with its extensive
comment.

Therefore move lldpad_wait() to lib.sh in order to allow reuse. Rename
it to lldpad_app_wait_set() to recognize that what this is intended to
wait on are the pending APP sets.

For the sleeping, add a function lldpad_app_wait_del(). That will serve
to hold the related explanatory comment (which edit for clarity), and as
a token in the caller to identify the sites where this sort of waiting
takes place. That will serve when/if a better way to handle this
business is found.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>