add test to support 7484fbbdfe8633578be15e; see #631
Math mode should be always enabled for inline numbers in LaTeX

Reasons:

- Negative numbers get a proper "minus" sign (not a dash)
- Old-style figures can be enabled for a document, but usually aren't for math mode (cf. texdoc newtxdoc)
- Special characters such as \infty (cf. #629)
Merge branch 'ensuremath-always' into HEAD
 #629: failing test for Inf as inline value; test NaN, too