Add composite LinearStretch test
Add composite LinearStretch test

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