acpt: add set shape fill transparent scenario
text: add _Font.fill
dml: refactor FillFormat test fixtures

parameterize tests on xPr (EG_FillProperties) so the same set can be
run on rPr, spPr, defRPr, etc.
text: convert _FontColor to fill.fore_color

The _FontColor class and tests are no longer required after converting
_Font.color to _Font.fill.fore_color