Minor.

Makes reading failing runtime test stacktraces easier (by having fewer
goroutines to read) on machines where these gdb tests wouldn't have
ever run anyway.

Change-Id: I3fab0667e017f20ef3bf96a8cc4cfcc614d25b5c
Reviewed-on: https://go-review.googlesource.com/81575Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>