cmd/go/internal/fsys: use a root other than "." in Walk tests
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 <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Trust: Jay Conrod <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>
Run-TryBot: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Reviewed-by: Jay Conrod <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>
Reviewed-by: David du Colombier <ec8ae1e6620d4bab5e0e7aa2a6ae41aad63e3708@gmail.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

CAFxX/go,golang/go,CAFxX/go,CAFxX/go,nawawi/go,golang/go,nawawi/go,CAFxX/go,cloudflare/go,cloudflare/go,golang/go,cloudflare/go,golang/go,nawawi/go,cloudflare/go,cloudflare/go,golang/go,nawawi/go,CAFxX/go,golang/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,cloudflare/go,golang/go,nawawi/go,cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go