runtime: bump thread count slack for TestReadMetricsSched
This test is *still* flaky, but it appears to be just
mayMoreStackPreempt and the thread count *occasionally* exceeds the
original (and arbitrary) thread count slack by exactly 1.
Bump the thread count slack by one. We can investigate further and bump
it again if it continues to be a problem.
Fixes #75664.
Change-Id: I29c922bba6d2cc99a8c3bf5e04cc512d0694f7fa
Reviewed-on: https://go-review.googlesource.com/c/go/+/708868 Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>