Fix editor shadow appearing under the selected line background when horizontal scroll is active

[InspiredGuy]

Issue description

Now, when horizontal scroll is active (triggered by line length exceeding editor width), editor displays shadow on the left side of it. It is an inset shadow, which means it is displayed below the content (see box-shadow docs for details). This shadow is overlapped by active line background colour. On the default theme it's not visible, because it has background colour, which is almost transparent (with 0.07 alpha channel value). If you try it on a theme with rgb background color without alpha channel like Clouds, issue becomes visible (see screenshots below).

Fix description

The issue is fixed by moving the shadow to the :after pseudo-element, which comes as a last child of the container. It is rendered above all other children, which leads to shadow appearing correctly above the active line.

Before

After

This PR also updates README with a path to correct tests.html location.

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[codecov]

Codecov Report

Base: 86.11% // Head: 86.11% // No change to project coverage 👍

Coverage data is based on head (4327faa) compared to base (1e0b5a9).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5020   +/-   ##
=======================================
  Coverage   86.11%   86.11%           
=======================================
  Files         546      546           
  Lines       41627    41627           
  Branches     6555     6555           
=======================================
  Hits        35847    35847           
  Misses       5780     5780           

Flag	Coverage Δ
unittests	86.11% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/css/editor.css.js	100.00% <ø> (ø)

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