Simply choose an extremely unlikely path name in the test is fine.

Fixes #21013.

Change-Id: I56c0a1986b5ef5d618c7fe2b14701f584fe81c37
Reviewed-on: https://go-review.googlesource.com/81578
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>