selftests: net: replace sleeps in fcnal-test with waits

fcnal-test.sh already includes lib.sh, use relevant helpers
instead of sleeping. Replace sleep after starting nettest
as a server with wait_local_port_listen.

Reviewed-by: David Ahern <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: kuba-moo