Additional unit tests for FileSystemWatcher

I've implemented some additional unit tests for ``FileSystemWatcher`` to verify that it's working as expected after building Mono. Some users on the ``freebsd-mono`` mailing list reported that upgrading from Mono 3.4 to 3.6+ caused some of their applications not to work any longer; in trying to diagnose the issue, I put together some simple tests to check whether ``FileSystemWatcher`` events are fired in the expected way. 

I developed these tests in Visual Studio on Windows/.NET 4.5, and all of them pass there. However, when the tests run on Mono (via ``make-check``) on FreeBSD 10 and Ubuntu 12.04, some/all of them fail. I haven't tried running them on OS X, but OS X uses the same kqueue-based implementation for ``FileSystemWatcher`` as FreeBSD, so I suspect their results will be similar.