Add unit tests to flexbox layout algorithm

[alice-i-cecile]

Now that the algorithm has been broken down into chunks in #88; we can start unit testing each of the parts.

This is essential for #39, and helps us decide what we want to do about the generated tests in #65.

If possible, I'd like to fuzz these (or use property-based testing): edge cases are hard.

[TimJentzsch]

Here are the specs for reference: https://www.w3.org/TR/css-flexbox-1/#layout-algorithm The individual functions also link to the specs.

[Weibye]

Based on the current state of taffy, it appears that unit-testing individual pieces of the algorithm is difficult and that we instead should rely on the current method of testing.

In which case this issue should be closed and then we can open more specific issues if we spot a part of the code that allows being unit-tested.