[testing] Fix path relativization on Windows
[testing] Fix path relativization on Windows

Change-Id: Ie685fb3cebf9e9c638c1e82b0f88d1c540c86a98
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/143810
Reviewed-by: Jens Johansen <05223a3c6bdf76af6f84755eae231abeebcf4530@google.com>
Commit-Queue: Johnni Winther <0f64ef784d57ffb064b634040f9eeba0d33d0be4@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk