Add a MathML test for italic mi with border, padding and margin (#47052)
padding-border-margin-002.html and padding-border-margin-003.html
verify handling of border, padding and margin on a stretchy or largeop
<mo> elements, since browsers implement special layout/painting for
them. This patch verifies handling of border, padding and margin on a
single-char <mi> (i.e. that picks italic mathvariant [1]) comparing it
to the an <mtext> element with the transformed character. That covers
WebKit's special layout/painting for this case [2].
[1] https://w3c.github.io/mathml-core/#the-mi-element
[2] https://bugs.webkit.org/show_bug.cgi?id=276317