runtime: use DontUpdate testprog for DontUpdate test

runtime: use DontUpdate testprog for DontUpdate test

This is a typo in CL 670497. The test is using the wrong testprog
function.

The testprog also needs to assert that GOMAXPROCS doesn't change, not
that it is equal to NumCPU, for the GOMAXPROCS=4 case.

For #73193.

Cq-Include-Trybots: luci.golang.try:gotip-windows-amd64-longtest
Change-Id: I6a6a636cab6936aa8519e3553b70ab6641ca8010
Reviewed-on: https://go-review.googlesource.com/c/go/+/675097
Auto-Submit: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>