[asdf-devel] Added a new test

I originally thought that ASDF might not notice if a source file was lost, as long as a fasl was left around. In fact, it /does/ notice, which is what I had hoped. I just added a test to verify this, just in case. Please try and run this when you get a chance, just to make sure it's truly portable. I'm particularly curious about whether it will work on Windows. thanks, r