runtime: skip TestCgoCallbackGC on darwin-amd64-10_14 builder

This test occasionally fails due to a real bug on this platform.
Due to the age of the platform and the rarity of the failure, we do
not believe that the bug is worth working around.

Fixes #43926

Change-Id: Ia227c5afe81fc21b6630813228f976cc3a54013c
Reviewed-on: https://go-review.googlesource.com/c/go/+/366537


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>