To re-add opted out contacts to a texting campaign, contacts have to text your Keyword to 313131 in the US or 393939 in Canada. This includes your number if you've opted out of your own test text message.
To find your keyword, you want to go to Keywords on the left navigation panel. All accounts have at least one demo keyword that can be used for your contacts (and you) to opt back in to a texting list. Demo keywords start with the letters "ez" and are completely free to use.