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I have tried to submit a clone version of your form that I have created, I have integrated Authorize.Net in sandbox(test) mode. I was able to see the Thank You page after trying to make a test submission.
Here is the guide on How to Test Authorize.net Payment Tool on Sandbox Mode?
This might also help you in testing: http://developer.authorize.net/hello_world/testing_guide/
Let us know if you need any further assistance. Thank you.