runtime: relax threadsSlack in TestReadMetricsSched

runtime: relax threadsSlack in TestReadMetricsSched

runtime.GC is called in the test and may spin up GOMAXPROCS
(proportional to the initial count) new threads. We need to be robust to
this, and it happens relatively frequently on some platforms.

We didn't notice this earlier since the heap is so miniscule that
runtime.GC essentially finished instantly without all the threads
getting spun up.

Fixes #76613.

Change-Id: I2af02cb090d5c1c952e4db53ad35895b6f23f638
Reviewed-on: https://go-review.googlesource.com/c/go/+/726642
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>