io: add test for Pipe constructor allocations

io: add test for Pipe constructor allocations

Updates #67633

Change-Id: If3da9317ba36cb8a7868db94b45c402e1793e018
Reviewed-on: https://go-review.googlesource.com/c/go/+/588219
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Mateusz Poliwczak <mpoliwczak34@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Than McIntosh <thanm@google.com>