Having been caught out recently when a web site I launched displayed perfectly on IE, Firefox, Chrome and Safari on Windows but was corrupted when viewed using Safari on the Mac
Unfortunately you cannot run MacOS X on anything but a genuine Mac.
MacOS X Server however can be run in VMWare. A stopgap solution would be to install it inside a VM. But you should be aware that MacOS X Server and MacOS X are not exactly the same, and your testing is not going to be exactly what the user has. Not to mention the $499 price tag.
Simplest way is to buy yourself a cheap mac mini or a laptop with a broken screen used on ebay, plug it onto your network and access it via VNC to do your testing.