Added text tessellation test
Set OPENSCAD_FONT_PATH also for the normal test cases.

This was already set for the export/import test cases as the exported
CSG files do not have the use<fontfile> statements. Due to differences
in the glyphs between the Liberation 2.00.1 we ship and the system
installed fonts some test cases (e.g. text_on_cube) did also fail.
#1006 Added testcase
Merge remote-tracking branch 'origin/master' into externalize-colorschemes2