runtime: skip TestCPUMetricsSleep as flaky

runtime: skip TestCPUMetricsSleep as flaky

This test is fundamentally flaky because of a mismatch between how
internal idle time is calculated and how the test expects it to be
calculated. It's unclear how to resolve this mismatch, given that it's
perfectly valid for a goroutine to remain asleep while background
goroutines (e.g. the scavenger) run. In practice, we might be able to
set some generous lower-bound, but until we can confirm that on the
affected platforms, skip the test as flaky unconditionally.

For #60276.
For #60376.

Change-Id: Iffd5c4be10cf8ae8a6c285b61fcc9173235fbb2a
Reviewed-on: https://go-review.googlesource.com/c/go/+/497876
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>