io.tests: added test of inherited registration
io.tests: added test of inherited registration

see astropy/astropy#7156

dhomeier/astropy,larrybradley/astropy,stargaser/astropy,MSeifert04/astropy,aleksandr-bakanov/astropy,pllim/astropy,StuartLittlefair/astropy,aleksandr-bakanov/astropy,pllim/astropy,StuartLittlefair/astropy,astropy/astropy,MSeifert04/astropy,lpsinger/astropy,funbaker/astropy,DougBurke/astropy,stargaser/astropy,funbaker/astropy,funbaker/astropy,DougBurke/astropy,saimn/astropy,StuartLittlefair/astropy,lpsinger/astropy,larrybradley/astropy,aleksandr-bakanov/astropy,funbaker/astropy,astropy/astropy,astropy/astropy,MSeifert04/astropy,saimn/astropy,lpsinger/astropy,StuartLittlefair/astropy,bsipocz/astropy,stargaser/astropy,mhvk/astropy,MSeifert04/astropy,pllim/astropy,saimn/astropy,mhvk/astropy,mhvk/astropy,stargaser/astropy,StuartLittlefair/astropy,DougBurke/astropy,dhomeier/astropy,lpsinger/astropy,aleksandr-bakanov/astropy,larrybradley/astropy,lpsinger/astropy,dhomeier/astropy,dhomeier/astropy,astropy/astropy,pllim/astropy,astropy/astropy,larrybradley/astropy,bsipocz/astropy,saimn/astropy,dhomeier/astropy,saimn/astropy,mhvk/astropy,bsipocz/astropy,bsipocz/astropy,DougBurke/astropy,larrybradley/astropy,mhvk/astropy,pllim/astropy