Run test suite on Windows

[bgilbert]

CI performs a Windows build as of #366, but we should also run tests, ideally on actual Windows rather than Wine. The old Buildbot setup ran tests after building on Windows directly, but we presumably want to stick with Linux cross builds (xref openslide/openslide-bin#36) so this will require some novel plumbing. Perhaps windows.yml will need to archive the winbuild root tree for us.

The right approach may be to wait for #316 rather than adding even more glue.