paygen: Unless told otherwise, use au generator version 6351.0.0 for testing.

That version is compatible with ToT chromite (for now) and will get the
network tests to pass again. We'll want to use ToT versions of the utilities
in the au generator zip in the future to ensure compatibility, but that will
require more extensive changes.

This minimal pinning only applies to the unit tests which use an old version
for the target image. In production, the target image is generally the one we
just built and so has an up to date au generator zip.

BUG=chromium:470340
TEST=Ran the paygen_payload_lib_unittest with --network and saw that it now
passes.

Change-Id: Ifb3acffbeb16ebc12ef5331f9a8176fb6dad0c5a
Reviewed-on: https://chromium-review.googlesource.com/262772
Reviewed-by: Alex Deymo <deymo@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>
Commit-Queue: Gabe Black <gabeblack@chromium.org>
Tested-by: Gabe Black <gabeblack@chromium.org>
1 file changed