Fixed a test failure that occurs when the specified font is not installed

On systems where 'Liberation Sans' is not installed out-of-the-box (e.g. Ubuntu Server, OpenSuse Server) it falls back to 'DejaVu Sans', which makes the GetHashCode_NameDiffers_HashesNotEqual() test fail as the two font names are now equal. Instead we use 'Arial' and 'Courier New' which resolve to different fonts on most systems.

Note: I changed the other two tests that used 'Liberation Sans' too, so they are consistent with the rest of the font test suite which uses 'Arial'.