Ensure that root flexbox node sizes are floored by their padding border

[nicoburns]

Objective

• Fixes Uses the sum of border and padding if it's greater than the container definite size #651

Notes

• This bug only seems to affect nodes that happen to be the root of the tree.

Changes made from #651

• Test moved to tests/root_constraints
• Fix changed so that override does not apply to known_dimensions
• A generated test covering this case added
• Removed import of Add trait (uses + syntax rather than calling .add so it isn't needed)