Also removes unnecessary test cases for avoiding unexpected failures on
newer operating systems.

Updates #14724.

Change-Id: I2291585d951fb70383da68293a6ac1ff3524c7f7
Reviewed-on: https://go-review.googlesource.com/22452
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>