syscall: move helper handler before AIX handler in TestPassFD

syscall: move helper handler before AIX handler in TestPassFD

The AIX special handler which skips this test if unix network isn't
supported, doesn't need to be called inside the helper process.

Change-Id: I7ff2c4e6b20eceb977380294858cae63034ffe0d
Reviewed-on: https://go-review.googlesource.com/c/go/+/172160
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>