[ASDisplayNode] Add unit tests for layout z-order changes (with an open issue to fix).

[appleguy]

Right now, automatic subnode management is not fixing up the z-order for nodes and their views, whenever existing subnodes are moved to a different place in the layout.

This can result in particularly weird issues when an existing layout changes with some additional nodes and some removed nodes that may displace or separate some existing nodes that also need to change order.

Once we fix, it will be easy to improve the Overlay and Background tests that are below this to also check the reversal / relayout case (I don't think they would pass currently). cc @maicki @nguyenhuy @Adlai-Holler

ASDisplayNodeTests.mm:2410: error: -[ASDisplayNodeTests testThatStackSpecOrdersSubnodesCorrectly] : (([node.subnodes isEqualToArray:nodesReverseOrder]) is true) failed - subnodes: (
    "<ASDisplayNode: 0x7f8281101f90; a>",
    "<ASDisplayNode: 0x7f8281105b40; b>",
    "<ASDisplayNode: 0x7f8281105e80; c>",
    "<ASDisplayNode: 0x7f82811061c0; d>"
), reference array: (
    "<ASDisplayNode: 0x7f82811061c0; d>",
    "<ASDisplayNode: 0x7f8281105e80; c>",
    "<ASDisplayNode: 0x7f8281105b40; b>",
    "<ASDisplayNode: 0x7f8281101f90; a>"
)
Test Case '-[ASDisplayNodeTests testThatStackSpecOrdersSubnodesCorrectly]' failed (0.006 seconds).

[appleguy]

Without these mutable index sets and checking that all values were consumed, the test actually didn't fail when I tried deleting one of the supposed-to-be-present index values from the test set.

So although everything seems to behave correctly, this was important to ensure the validation checks both necessity and sufficiency of the test set.

[nguyenhuy]

LGTM. Thank you!

[maicki]

LGTM too.

I remember we had plans on implementing it soon, but unfortunately time and other stuff came in between ...