[asdf-devel] Added a new test

[Robert Goldman]

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