Fix problem with \vert becoming stretchy after enrichment. (mathjax/MathJax#3530)

[dpvc]

This PR fixes a problem with \vert losing its stretchy="false" attribute when in infix form (where this is the default), but after semantic enrichment, being in pre- or postfix position (where it is not). This is similar to #1426, which handled explicit | characters, but missed handling \vert. The tests were updated to account for the difference.

An alternative to this (and #1426) would be to adjust the filter that removes redundant attributes to leave explicit stretchy attributes when the mo content is |. If you prefer that, I will redo this PR.

Resolves issue mathjax/MathJax#3530

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.49%. Comparing base (5caee44) to head (67e1d02).

Additional details and impacted files

@@            Coverage Diff             @@
##           develop    #1440     +/-   ##
==========================================
  Coverage    86.49%   86.49%             
==========================================
  Files          340      340             
  Lines        86044    86047      +3     
  Branches      4833     3191   -1642     
==========================================
+ Hits         74425    74428      +3     
- Misses       11595    11619     +24     
+ Partials        24        0     -24     

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