selftests: forwarding: add wait_for_dev() helper

The existing setup_wait*() helper family check the status of the
interface to be up. Introduce wait_for_dev() to wait for the netdevice
to appear, for example after test script does manual device bind.

Signed-off-by: Jiri Pirko <[e-mail]>
Reviewed-by: Petr Machata <[e-mail]>
Reviewed-by: Benjamin Poirier <[e-mail]>
Signed-off-by: Paolo Abeni <[e-mail]>

tools/testing/selftests/net/forwarding/lib.sh

···
 	sleep $WAIT_TIME
 }
 
+wait_for_dev()
+{
+        local dev=$1; shift
+        local timeout=${1:-$WAIT_TIMEOUT}; shift
+
+        slowwait $timeout ip link show dev $dev &> /dev/null
+        if (( $? )); then
+                check_err 1
+                log_test wait_for_dev "Interface $dev did not appear."
+                exit $EXIT_STATUS
+        fi
+}
+
 cmd_jq()
 {
 	local cmd=$1