[test.py] Fix output path so it also works on Windows (take 2)
[test.py] Fix output path so it also works on Windows (take 2)

A path like
E:\b\build\slave\[...]\short27_al_args_concrete_subclass_test\E:\b\[...]
is not going to work on Windows, and while a path like
/whatnot/path/blah//whatnot/path
does work in Linux it doesn't look great.
This should fix it (somewhere else than before).

Bug:
Change-Id: I3e926ceb5a5e510b411328872ddc5da9efdf9f2e
Reviewed-on: https://dart-review.googlesource.com/27740
Reviewed-by: Kevin Millikin <a6737c91ef81c790d8ad0ae8762bd5b2332a6e42@google.com>

dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk