Add a few more tests suggested by @astrofrog
Add a few more tests suggested by @astrofrog

* Test H0, K as quantities.
* Test FLRW.de_density scale directly.
* Test FlatwCDM

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