Bug 1729450 - Fix the text-emphasis-position-* testcases/references to render...

Bug 1729450 - Fix the text-emphasis-position-* testcases/references to render consistently. r=emilio

The text-emphasis-position testcases do not match the current spec, as they use the keywords 'above'
and 'below', whereas CSS Text Decorations actually specifies 'over' and 'under'. (I guess they were
created before the values were finalized.)

We also need to convert the reference lines from using abs-pos to try and simulate emphasis mark rendering
to a ruby-based version, as this is how the spec defines the rendering of the marks. The abs-pos approach
is at best an approximation, and will result in spurious reftest failures even when the emphasis properties
are working as intended.

Depends on D124793

Differential Revision: https://phabricator.services.mozilla.com/D124794