os: enable TestClosedPipeRace* on FreeBSD

This test has worked since CL 165801 (committed March 12, 2019), so
stop skipping it. With this, we check that Close makes concurrent I/O
operations on pipes return Errclosed on all platforms.

Updates #19093.

Change-Id: Ic090c70996c115abf80d8f9b93ca2aeaf347c9d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/371016


Trust: Austin Clements <austin@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>