autotests: client -> window

Most of the MR was created with the help of grep and clang-rename, e.g.

  for file in (find autotests/integration -iname '*.cpp')
      for offset in (grep -bo "auto c = Test::render" $file | tac | cut -f1 -d:)
          clang-rename -i -offset=(math $offset + 5) -new-name=window $file
      end
  end

In a few places, I had to rename variables manually in order to resolve name conflicts.