fix the axislabels_regression test
fix the axislabels_regression test

Use all the bboxes for all the axes to calculate label position.

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