plot: test glyphboxes for plain and latex handlers

working on https://github.com/gonum/plot/pull/708 uncovered a slew of bugs related to `plot.GlyphBox{,er}`.
we should probably test more thoroughly the various modes in which `plot.GlyphBox`es are involved:

- [ ] with a `text.Plain` handler (the default)
- [ ] with a `text.Latex` handler
- [ ] with and without a `plot.Plot.Title`
- [ ] with and without a `plot.Plot.{X,Y}.Label`
- [ ] with and without rotated labels, title and legend entries (#722, #724)
- [ ] with and without a centered `plot.Plot.{X,Y}.Label` (+ rotated or not?) (#723)