All correspondence should now be in writing (or at least email) so that you have a record of anything said and any accusations made. People will be cautious of accusing you of anything in writing. If things go badly, you can copy your emails to a senior person and be open about that.
Never say anything in anger and never accuse or criticise. Simply state the facts in a dispassionate way.
For example
You could offer in writing to take a similar test whilst being observed.
It is unlikely that you will be taken up on this but the fact that you have offered will make them think twice.
If there is any disciplinary action, you can then point out that you were willing to demonstrate your strategy and your ability. Of course you have to make sure that the new test is not made especially hard, so specify "at the same level of difficulty".
Important
Never act in the heat of the moment. Take your time, stay polite but be insistent. Do not give up.
Example email
Dear X
With regard to our conversation about possible cheating.
I note that the automated timing system may have indicated something unusual. My exam strategy is to hurry through all the questions and then return to check my answers. In this case the initial stage took me about 15 minutes and the checking stage took a further x minutes. I checked thoroughly but did not need to correct any answers.
If there is any doubt at all of my skills or ability I will be willing to take a further such test (at the same level) under supervision
I am willing to take such a test online or manually with an observer present.
Nan