XdgShell and WlShell behave very differently when it comes to switching
from normal to maximised to fullscreen and back. Under XDGShell they are
2 properties, under WlShell it's a tristate enum.

This test was testing something very specific under WlShell and then
became a horrid mess of if statements doing different things and testing
different things. Especially after XdgShell got proper configure
handling.

This patch splits it into two methods.

Ran test
Passed