runtime: make use of builtin clear in tests

runtime: make use of builtin clear in tests

This is a follow-up to CL 574675.

Change-Id: I98c3ea968e9c7dc61472849c385a1e697568aa30
Reviewed-on: https://go-review.googlesource.com/c/go/+/574975
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Jes Cok <xigua67damn@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>