font/sfnt: increase proprietary_test coverage.
font/sfnt: increase proprietary_test coverage.

Recent commits (probably 792d36e1 "support TrueType compound glyphs" and
1995ed1a "support TrueType glyph transformations") let us decode all the
glyphs in the Adobe TrueType fonts, but I forgot to update the test
coverage in those commits.

Change-Id: I7b135005cd67082f84aee16d296debdedcc9488e
Reviewed-on: https://go-review.googlesource.com/38281
Reviewed-by: David Crawshaw <a2f0bf29257797dd66800b4fbb9945c3e216c004@golang.org>

golang/image