internal/cpu: revise test to make it work properly with -cover

internal/cpu: revise test to make it work properly with -cover

Fix up a test to insure that it does the right thing when
"go test -cover" is in effect.

Fixes #52761.

Change-Id: I0c141181e2dcaefd592fb04813f812f2800511da
Reviewed-on: https://go-review.googlesource.com/c/go/+/404715
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>