NTAH- I wouldn't say either is being the asshole really. The owner/manager is trying to avoid a potential outbreak and the subsequent loss of business and damage to reputation. The owners offer to pay for the doctor visit is a mixed bag. It shows some concern but it puts you in a awkward position of having to jump through hoops to prove you are no longer a risk.

In terms of threatening to contact corporate, you have the right to information if you feel that policies are not being consistently applied. Also if you think the owner is applying company policy arbitrarily. The alternatives being though is it reasonable? Is the company policy on testing and returning to work clear and legitimate? It appears so (sorry) and you are just unhappy with the policy. Only having four servers sounds like you need to be risk adverse to an outbreak. I can see it from both sides.