Also new tests added. So, perhaps, this CL corrects
even more broken EvalSymlinks behaviour.

Fixes #12451

Change-Id: I81b9d92bab74bcb8eca6db6633546982fe5cec87
Reviewed-on: https://go-review.googlesource.com/16192Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>