cmd/go/internal/fsys: use a root other than "." in Walk tests
Fixes #42115
Change-Id: Icf4c9eac5ed3295acbc8377c7a06f82c6bddc747
Reviewed-on: https://go-review.googlesource.com/c/go/+/264177
Trust: Bryan C. Mills <bcmills@google.com>
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com> Reviewed-by: Jay Conrod <jayconrod@google.com> Reviewed-by: David du Colombier <0intro@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>