path/filepath: deflake TestEvalSymlinksAboveRoot on darwin

On darwin, under load, it appears that the system occasionally
deletes the temp dir mid-test. Don't fail the test when that happens.

It would be nice to fix this in a deeper way.
See golang.org/cl/332009 for some discussion.

In the meantime, this will at least stop the flakiness.

Updates #37910

Change-Id: I6669e466fed9abda4a87ca88345c04cd7986b41e
Reviewed-on: https://go-review.googlesource.com/c/go/+/332009


Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>