os: permit RemoveAll with paths that end in ".."

Prohibiting RemoveAll with paths that end in ".." was added with
CL 137442 in this release cycle, but it worked before and it should
continue to work.

Also run TestRemoveAllDot on all systems; the test is not specific to
the use of unlinkat and friends.

Change-Id: I277784c8915cd748fec318d2936062440d5d1fde
Reviewed-on: https://go-review.googlesource.com/c/150497


Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>