Follow-up task to T375974 with other minor problems related to operators.

See https://www.mediawiki.org/wiki/Extension:Math/Native_MathML/Operators

• <math>B * C \ast D</math> the rendering of * and \ast is different in mathml, but not in SVG. Case 12e
• <math>A \setminus B \smallsetminus B \times C</math> \setminus and \smallsetminus render differently in SVG, but not in MathML. Case 12l
• <math>A \nsubseteqq B \nsupseteqq C</math> render differently, in SVG and MthML. Cases 12q and 12r
• <math>A := B</math> space added between : and = in MathML mode bug T401746, case 12t
• <math>E \nleq F \nleqq G</math> both operators render identically in MathML but differ in SVG, case 12y
• <math>E \ngeq F \ngeqq G</math> both operators render identically in MathML but differ in SVG, case 12z
• <math>A \parallel B \nparallel B \shortparallel B \nshortparallel C</math> short versions are shorter in SVG but same size in MathML
• <math>A \ulcorner B \urcorner B \llcorner B \lrcorner C</math> more spaces around operators in MathML than SVG.