Add tests for allowed_parent_page_models and allowed_subpage_models.
Add tests for allowed_parent_page_models and allowed_subpage_models.

We don't want to only test these indirectly via allowed_*_types, because
allowed_*_models is the preferred interface.

zerolab/wagtail,Toshakins/wagtail,FlipperPA/wagtail,FlipperPA/wagtail,thenewguy/wagtail,kurtrwall/wagtail,gasman/wagtail,mixxorz/wagtail,rsalmaso/wagtail,hamsterbacke23/wagtail,nilnvoid/wagtail,davecranwell/wagtail,gasman/wagtail,mikedingjan/wagtail,kurtrwall/wagtail,serzans/wagtail,gasman/wagtail,nimasmi/wagtail,timorieber/wagtail,jnns/wagtail,quru/wagtail,wagtail/wagtail,gasman/wagtail,davecranwell/wagtail,nealtodd/wagtail,hamsterbacke23/wagtail,nealtodd/wagtail,takeflight/wagtail,mikedingjan/wagtail,gogobook/wagtail,kaedroho/wagtail,davecranwell/wagtail,JoshBarr/wagtail,kurtw/wagtail,nilnvoid/wagtail,iansprice/wagtail,chrxr/wagtail,kurtw/wagtail,zerolab/wagtail,gasman/wagtail,nutztherookie/wagtail,kaedroho/wagtail,inonit/wagtail,nutztherookie/wagtail,torchbox/wagtail,nilnvoid/wagtail,rsalmaso/wagtail,nimasmi/wagtail,wagtail/wagtail,inonit/wagtail,thenewguy/wagtail,zerolab/wagtail,nimasmi/wagtail,kurtrwall/wagtail,hamsterbacke23/wagtail,timorieber/wagtail,rsalmaso/wagtail,takeflight/wagtail,torchbox/wagtail,jnns/wagtail,FlipperPA/wagtail,serzans/wagtail,gogobook/wagtail,rsalmaso/wagtail,Toshakins/wagtail,nilnvoid/wagtail,gogobook/wagtail,JoshBarr/wagtail,inonit/wagtail,iansprice/wagtail,davecranwell/wagtail,quru/wagtail,nutztherookie/wagtail,nimasmi/wagtail,kaedroho/wagtail,torchbox/wagtail,nealtodd/wagtail,nutztherookie/wagtail,jnns/wagtail,inonit/wagtail,wagtail/wagtail,zerolab/wagtail,jnns/wagtail,Toshakins/wagtail,timorieber/wagtail,kurtw/wagtail,hamsterbacke23/wagtail,mixxorz/wagtail,FlipperPA/wagtail,rsalmaso/wagtail,torchbox/wagtail,takeflight/wagtail,kaedroho/wagtail,kurtrwall/wagtail,wagtail/wagtail,thenewguy/wagtail,timorieber/wagtail,JoshBarr/wagtail,mixxorz/wagtail,JoshBarr/wagtail,kaedroho/wagtail,mixxorz/wagtail,chrxr/wagtail,serzans/wagtail,Toshakins/wagtail,chrxr/wagtail,mixxorz/wagtail,takeflight/wagtail,quru/wagtail,quru/wagtail,kurtw/wagtail,nealtodd/wagtail,thenewguy/wagtail,mikedingjan/wagtail,serzans/wagtail,chrxr/wagtail,mikedingjan/wagtail,wagtail/wagtail,thenewguy/wagtail,zerolab/wagtail,iansprice/wagtail,gogobook/wagtail,iansprice/wagtail