I'm trying to set up a Test Driven Development environment and I was wondering if it's possible to run the WordPress test suite without a web server. I'm on a mac (10.7.5) and I've set up phpunit and the mysql server.
You're on a Mac! You have two of the three components you need for a Web server by default. Apache and PHP. All you need to do now is get MySQL. There are plenty of tutorials on how to do this and can be found with Google.
Of course NW Tech recommendation of MAMP is another great option. Especially if you haven't managed a server before. Check out the WordPress Codex.