Add a :window-only for some qt5 end2end tests

A couple of tests seem to be failing because there is two windows open and
they don't expect it. I haven't fixed the root cause, I looked through the
logs and couldn't see why a second window was open. But if this makes the
tests pass, I guess we can address that if someone reports it.

Also changed `_get_scroll_values()` to handle multiple tabs/windows being open
too while I was there.

Example logs: https://github.com/qutebrowser/qutebrowser/actions/runs/8548730512/job/23422922448
Sessions file from the test logs:

  Current session data:
  windows:
  - geometry: !!binary |
      AdnQywADAAAAAAAyAAAAMgAAA1EAAAKJAAAAMgAAADIAAANRAAACiQAAAAAAAAAAAyAAAAAyAAAA
      MgAAA1EAAAKJ
    tabs:
    - active: true
      history:
      - active: true
        last_visited: '2024-04-04T03:21:00'
        pinned: false
        scroll-pos:
          x: 0
          y: 0
        title: about:blank
        url: about:blank
        zoom: 1.0
  - active: true
    geometry: !!binary |
      AdnQywADAAAAAAAyAAAAMgAAA1EAAAKJAAAAMgAAADIAAANRAAACiQAAAAAAAAAAAyAAAAAyAAAA
      MgAAA1EAAAKJ
    tabs:
    - active: true
      history:
      - active: true
        last_visited: '2024-04-04T03:21:03'
        pinned: false
        scroll-pos:
          x: 0
          y: 40
        title: Scrolling
        url: http://localhost:39235/data/scroll/simple.html
        zoom: 1.0

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