runtime: skip windows stack tests in race mode

runtime: skip windows stack tests in race mode

These became race instrumented in CL 643897, but race mode uses more
memory, so the test doesn't make much sense.

For #71395.

Change-Id: I6a6a636cf09ba29625aa9a22550314845fb2e611
Reviewed-on: https://go-review.googlesource.com/c/go/+/675077
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>