path/filepath: add test cases for walking a symlink-to-symlink-to-dir

path/filepath: add test cases for walking a symlink-to-symlink-to-dir

The "double link with slash" test is skipped on darwin due to an
apparent kernel / libc bug. If the bug is present on other platforms
too, I'd like to know about it.

For #59586.

Change-Id: I4bdd6a80a3ed7b0960ea6da30f91a655f317d512
Reviewed-on: https://go-review.googlesource.com/c/go/+/484395
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>