This recent post from the Mind Tree blog shares a few methods for testing your web application (not unit test, just general things).

Because the Web "environment" is so diverse and contains so many forms of programmatic content, input validation and sanity checking is the key to Web applications security. This involves both identifying and enforcing the valid domain of every user-definable data element, as well as a sufficient understanding of the source of all data elements to determine what is potentially user definable.

They note that the root of most problems is input validation - most applications either just don't do it or do it poorly. They include a few tips on first security the environment the application is running in (like checking the HEAD/OPTIONS values and ensuring you're only allowing known file extensions and directories). They also mention the insecurity behind HIDDEN form elements and some issues surrounding user authentication.