For NetCommunity versions after 6.53: 

Test messages should send immediately. If the test message hasn't sent, please refer to: Email stuck in NetCommunity.
Note: If there are any large messages sending out at the same time, these could delay the test message from sending out until the other message has completed. 

For NetCommunity versions prior to 6.53:

In order to send a test message, verify the Send After date is not set in the future. If the send after date is set for the future, the test message will not send until that time. Change the send after date and send another test.