Spurious failure in SBCL on windows

[Robert Goldman]

When I run test all on Windows, the SBCL tests fail in a spurious way.

I get an error that lisp couldn't rename f:\asdf\build\dir1\foo0.asd to
f:\asdf\build\dir1\foo1.asd.bak

AFAICT this is because CREATE-ASD-FILES in test-configuration.script
uses :if-exists :rename-and-delete when it's creating the .asd files.

Is this actually necessary?  Could we replace this with :supersede, or
will something go awry?  For now I'm experimenting with that
replacement, so please just LMK if this is A Bad Thing.

thanks,
r