Limit number of times a test can be taken

You can do that with a variable.

Assuming you have an Introduction screen to your assessment:

Create a True/False Variable and name it TestTaken

Now, edit the Trigger on the "Next" button on the Introduction screen by adding a Condition" If TestTaken = value False (this statement will check if the variable is True or False and if False, it will jump to next screen. If it's True, it wont. 

Then put a Trigger on the Results screen of your Assessment that changes the TestTaken variable to True, Adjust Variable TestTaken to True when the timeline starts.

So what happens is that the first time the user reaches the Assessment, the variable TestTaken is False so the Next button allows them to go to the next screen. Once they finish the Assessment and reach the Results screen, that variable is changed to True. If they go back to the Introduction of the Assessment screen and click Next, the course wont advance because the statement is True. You have to decide what happens when the Statement is "True". You coulds end them to another screen or load a Layer that has text that says they've already taken the test.