bytes: skip TestNewBufferShallow if optimization is disabled

bytes: skip TestNewBufferShallow if optimization is disabled

TestNewBufferShallow should be skipped if optimization is disabled.

It is currently failing on no-opt builders.

Change-Id: Ib5e62022a56a4e5f158f247d69a6229d2cb4d99e
Reviewed-on: https://go-review.googlesource.com/c/go/+/581915
Auto-Submit: Than McIntosh <thanm@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Auto-Submit: Quim Muntal <quimmuntal@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Joedian Reid <joedian@google.com>