path/filepath: better error reporting during TestWinSplitListTestsAreValid
path/filepath: better error reporting during TestWinSplitListTestsAreValid

Fixes issue 4930.

R=golang-dev, minux.ma
CC=golang-dev
https://codereview.appspot.com/7424043
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go