bytes: add test to ensure shallow copy of NewBuffer does not allocate

bytes: add test to ensure shallow copy of NewBuffer does not allocate

At present, there is no API to reset the underlying []byte
of an existing Buffer struct, except to shallow copy
the entire Buffer struct.

Updates #67004

Change-Id: I08998f7a95ae5bde0897d86247d47f23cd784583
Reviewed-on: https://go-review.googlesource.com/c/go/+/581297
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Joedian Reid <joedian@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>