directory/testdir: refactor access checker to use WhichAccess
directory/testdir: refactor access checker to use WhichAccess

Much nicer code.

Change-Id: If3d10235547b33206c2afb7ffae9aa184c37843d
Reviewed-on: https://upspin-review.git.corp.google.com/1736
Reviewed-by: Eduardo Pinheiro <55040cbb39ba395339ae7940d8da6cd97797a4a8@google.com>

upspin/upspin,upspin/upspin