test: fix tests to work with sizespecializedmalloc turned off

test: fix tests to work with sizespecializedmalloc turned off

Cq-Include-Trybots: luci.golang.try:gotip-linux-386-nosizespecializedmalloc,gotip-linux-amd64-nosizespecializedmalloc,gotip-linux-arm64-nosizespecializedmalloc
Change-Id: I6a6a696465004b939c989afc058c4c3e1fb7134f
Reviewed-on: https://go-review.googlesource.com/c/go/+/720401
Auto-Submit: Michael Matloob <matloob@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@google.com>