unix: test (*FdSet).Clear in TestFdSet
unix: test (*FdSet).Clear in TestFdSet

Follow-up for CL 205397 so all *FdSet functionality is tested.

Change-Id: Ieb5dce6ad020ce1252cae7f3fe96ba6bf0069f5e
Reviewed-on: https://go-review.googlesource.com/c/sys/+/215241
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

multiplay/golang-sys,golang/sys,golang/sys,multiplay/golang-sys,golang/sys,multiplay/golang-sys,multiplay/golang-sys