The issue fixed is that most Lisps don't like
*default-pathname-defaults* being a logical-pathname.
Added a test for logical-pathname functionality;
it all works on CCL, and works better on other Lisps after the fix,
but there are still logical-pathname issues on SBCL, CLISP and more.
Also, a package cleanup of our test suite.

	CVS: Enter Log.  Lines beginning with `CVS:' are removed automatically
Added *load-preference-files* variable (which defaults to nil). This
controls whether or not the ASDF preference file machinery gets run.
The rest of the preference machinery will be removed from ASDF
sometime in the future.

Also added several new test files.