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

Locate the existing generated layout tests and their coverage for flexbox, grid, and leaf layout modes. Add negative-margin cases for each mode, then use the test results to identify and correct any layout failures; done means the new cases pass across all three modes.

Written by the indexing model from the issue text.