make sure the local build tests exercise the export logic

Summary:
Export doesn't really do much in the toplevel module, *except* that if
you chain rules together, subsequent rules should use the export dir of their
predecessor as their cwd. We were doing this correctly, but it wasn't being
tested.

Test Plan:
Confirm that this test case breaks if I remove the code from
LocalModule that respects export.

Reviewers: sean

Reviewed By: sean

Differential Revision: https://phabricator.buildinspace.com/D54