In fact, the [NSBundle mainBundle] when running a UnitTest is not the path of your app, but is /Developer/usr/bin, so this will not work.
The way to get resources in a unit test is here: OCUnit & NSBundle
In short, use:
[[NSBundle bundleForClass:[self class]] resourcePath]
or in your case:
[[NSBundle bundleForClass:[self class]] resourceURL]