runtime/pprof: disable TestMorestack on macOS under race detector

This is failing but only under the race detector.
It doesn't really seem fair to expect pprof to find
specific profile events with the race detector slowing
everything down anyway.

Change-Id: I4b353d3d63944c87884d117e07d119b2c7bf4684
Reviewed-on: https://go-review.googlesource.com/c/go/+/294071


Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>