Fix for `insertSubview` being off by one

[janek]

Fixes #308 Type of change: Bug fix

Motivation

We had a bug in insertSubview where we would use expected new positions in subview hierarchy before the hierarchy is updated. This fix uses the old hierarchy.

Visually in DemoApp it looks correct (where it previously was wrong). I also added a test that would fail on master, but passes here and on iOS.

Please check if the PR fulfills these requirements

• [x] Self-review: I am confident this is the simplest and clearest way to achieve the expected behaviour
• [x] There are no dependencies on other PRs or I have linked dependencies through Zenhub
• [x] The commit messages are clean and understandable
• [x] Tests for the changes have been added (for bug fixes / features)

[codecov[bot]]

Codecov Report

Merging #309 into master will increase coverage by 0.17%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #309      +/-   ##
==========================================
+ Coverage   51.97%   52.15%   +0.17%     
==========================================
  Files          87       87              
  Lines        3161     3160       -1     
==========================================
+ Hits         1643     1648       +5     
+ Misses       1518     1512       -6

Impacted Files	Coverage Δ
Sources/UIView.swift	90.68% <100%> (+0.93%)	:arrow_up:
Sources/CALayer.swift	88.43% <0%> (+2.72%)	:arrow_up:

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[janek]

@ephemer yes it does! thanks!