os: enable TestPipeThreads on FreeBSD
os: enable TestPipeThreads on FreeBSD

This test works on FreeBSD since CL 165801 was submitted.

Updates #19093

Change-Id: I45ffeb403c1de4385cdb21b9647f21976061e1ea
Reviewed-on: https://go-review.googlesource.com/c/go/+/373358
Trust: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

nawawi/go,cloudflare/go,cloudflare/go,CAFxX/go,CAFxX/go,nawawi/go,golang/go,cloudflare/go,golang/go,CAFxX/go,golang/go,nawawi/go,nawawi/go,golang/go,CAFxX/go,CAFxX/go,cloudflare/go,cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go,CAFxX/go,golang/go,nawawi/go,golang/go,golang/go,cloudflare/go,nawawi/go,cloudflare/go,CAFxX/go,nawawi/go,golang/go