plot: test glyphboxes for plain and latex handlers

[sbinet]

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.GlyphBoxes 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)