Add new tests for directory functions.

Test for DIRECTORY-FILES returning only files; text for SUBDIRECTORIES
returning only subdirectories.

According to Fare, DIRECTORY-FILES should *not* return subdirectories,
but it does on several implementations.  See
https://bugs.launchpad.net/asdf/+bug/1276748
Check for previously-existing copies of deleteme files.

At least on ABCL, having these previously-existing copies causes tests
to fail because of unexpected directory contents.
Avoid code loss by making the concatenated file read-only in emacs.
Fix for bug where DIRECTORY-FILES listed subdirectories.
Bump version to 3.1.0.64
Update TODO: more on cross-compilation, deferred-warnings, etc.