go/packages: remove POSIX filename assumption from test
go/packages: remove POSIX filename assumption from test

Change-Id: If1aac9f109b05897b21a2f27a4050fbee2cc2d2e
Reviewed-on: https://go-review.googlesource.com/123357
Run-TryBot: Alan Donovan <7069e9552f9d110b0a0f326b96f22f63fb99dfc1@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Michael Matloob <4562438b12c14e81a47550acde616ff328a909f8@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools