mathtext: Fix bugs in conversion of apostrophes to primes

[zblz]

• Fix mathtext mishandling of certain exponents #5864: wrong typesetting of $''/yr$ by recognising apostrophes in first position as part of the superscript.
• Fix the width of the superscript block when generated from apostrophes. For some reason the Hlist.extend method does not recompute the kerning (so in strings like $f''''(x)$ the primes would overrun the parens and the x), so I added an hpack() and kern() calls after extending.

I also modified an existing test to include the two bugs fixed.

[QuLogic]

This is really minor, but it seems like with the serif font, the f and single prime are slightly closer together than with triple primes. Is there some ligature invoked there?

[zblz]

@QuLogic - There is indeed a slight difference in the dejavuserif version. However, it is also present when typesetting it with \prime instead of apostrophes, so it is not related to the things done in this PR. Note that italics f are particularly troublesome with respect to kerning; see #4987.

[zblz]

ping @mdboom

[mdboom]

Backported to v1.5.x as cd866e9