I'm trying to create an MTurk HIT, it seems to work for the most part in the sandbox, however I cannot verify whether the submission logic works correctly because it seems like in the sandbox you cannot submit anything (this applies also to the examples given by Amazon), instead you get the following message:
There was a problem submitting your results for this HIT.
This HIT is still assigned to you. To try this HIT again, refresh the page. If this problem persists, you can contact the Requester for this HIT by clicking "HIT Details" above and then clicking "Contact This Requester" at the bottom of the pop up.
To return this HIT and continue working on other HITs, click the "Return" button on the top or bottom of the right side of the page.
Is there a better way other than actually publishing it for someone to try?
You can do complete end to end testing in the sandbox. This error you're seeing indicates that the submission isn't working correctly. You may encounter the same problem if you use the prod/non-sandbox endpoint, so you should figure out what's going wrong with the form submission.
A common problem is hard-coding the prod/non-sandbox endpoint in the form submission, so it doesn't correctly send it to sandbox when you are working HITs from the sandbox.