syscall: inline TestUseCgroupFDHelper

syscall: inline TestUseCgroupFDHelper

This way, the test case is self-contained, and the test output no longer
has useless lines related to TestUseCgroupFDHelper.

Change-Id: I575a03b678c8c08a87353f6dc00c35a90ec561ca
Reviewed-on: https://go-review.googlesource.com/c/go/+/525196
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>