[autotests] Untangle ShellClient::testMaximizedToFullscreen

Summary:
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.

Test Plan:
Ran test
Passed

Reviewers: KWin, zzag

Reviewed By: KWin, zzag

Subscribers: zzag, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D18589