Add tests for LayoutableChildren iterator

[NickGerleman]

Summary:
Adds unit tests that directly cover the order in which LayoutableChildren iterator goes over the descendant nodes. The covered cases are as follows (nodes with display: contents are marked green):

Single display: contents node

flowchart TD
R((R)) --> A((A))
R --> B((B))
R --> C((C))

B --> D((D))
B --> E((E))

style B fill:https://github.com/facebook/yoga/issues/090

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Correct order: A, D, E, C

Multiple display: contents nodes

flowchart TD
R((R)) --> A((A))
R --> B((B))
R --> C((C))

A --> D((D))
A --> E((E))

B --> F((F))
B --> G((G))

C --> H((H))
C --> I((I))

style A fill:https://github.com/facebook/yoga/issues/090
style B fill:https://github.com/facebook/yoga/issues/090
style C fill:https://github.com/facebook/yoga/issues/090

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Correct order: D, E, F, G, H, I

Nested display: contents nodes

flowchart TD
R((R)) --> A((A))
R --> B((B))
R --> C((C))

B --> D((D))
B --> E((E))

E --> F((F))
E --> G((G))

style B fill:https://github.com/facebook/yoga/issues/090
style E fill:https://github.com/facebook/yoga/issues/090

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Correct order: A, D, F, G, C

Leaf display: contents node

flowchart TD
R((R)) --> A((A))
R --> B((B))
R --> C((C))

style B fill:https://github.com/facebook/yoga/issues/090

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Correct order: A, C

Root display: contents node

flowchart TD
R((R)) --> A((A))
R --> B((B))
R --> C((C))

style R fill:https://github.com/facebook/yoga/issues/090

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Correct order: A, B, C - LayoutableChildren goes over the children with display: contents property, setting it on the root node should have no effect.

X-link: facebook/yoga#1731

Differential Revision: D64981779

Pulled By: NickGerleman

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D64981779

[cipolleschi]

landed in 33e1ae1