`xdg_toplevel_stable.cpp` cleanups

[tarek-y-ismail]

• Use StartedInProcessServer instead of InProcessServer
  • Should allow ConfigurationWindow to be pulled into a shared fixture instead of being duplicated in each test
• wlcs::Client client{the_server()}; can be moved into the fixture instead of being duplicated.
• Fullscreening an maximizing can use the utility functions introduced in Add tests to ensure states are properly preserved between unmaps and remaps #404