syscall: fixup TestUseCgroupFD error checking

syscall: fixup TestUseCgroupFD error checking

Fixup for CL 520265; of course errors returned from os/exec are wrapped.

While at it, change the order -- it seems more readable this way.

Change-Id: Ifb5d0c113f4fb2b3cc4be922021dbd2a8a886b7b
Reviewed-on: https://go-review.googlesource.com/c/go/+/524959
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>