os/exec: re-enable LookPathTest/16

This failure was confirmed to be due to a bug in prerelease versions
of Windows, and has been fixed by now. Remove the skip for this test.

Fixes #44379.

Change-Id: Idfb92ffd6b9d416d4c78ef3800a5ffdda06c6562
Reviewed-on: https://go-review.googlesource.com/c/go/+/341455


Trust: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>