enchant_providers/providers.test.cpp: replace obsolete macro
enchant_providers/providers.test.cpp: replace obsolete macro

Replace G_CONST_RETURN (which should not have been used anyway!) by const.

Also tidy up TestProvidersInDirectory a bit.

AbiWord/enchant,rrthomas/enchant,rrthomas/enchant,AbiWord/enchant,rrthomas/enchant,AbiWord/enchant,AbiWord/enchant,rrthomas/enchant