CSS:Tests: Fix tests & support tests under CSS Zoom (3.x version)

CSS:Tests: Fix tests & support tests under CSS Zoom (3.x version)

Firefox 126+ implements CSS zoom in a way it affects width computed style
very slightly (`100.008px` instead of `100px`); accept that difference.

Also, skip the width setter under zoom test in Firefox 126 completely - that
version has CSS zoom affecting `offsetWidth` values. This has been fixed in
Firefox 127 so it's not worth working around it.

Add a test for support tests resolving the same under CSS zoom & without one.
That test uncovered Chrome failing the `reliableTrDimensions` support test
under zoom; the test has been fixed.

Fixes gh-5489
Closes gh-5496
Ref gh-5495