Add tests for negative margins

What problem does this solve or what need does it fill?

CSS supports negative margins, and Taffy also support setting a negative value for a margin style. However Taffy currently doesn't have any tests for negative margins (and I suspect we don't lay them out correctly either, although we won't know until we have tests)

What solution would you like?

• Add generated tests for negative margins (for flexbox, grid, and leaf layout modes)
• Fix any bugs