Add tests for cross-split commands

[pokey]

What

We need to add tests to ensure that our actions work correctly when the target is in another split from the currently focused editor.

Which tests

• Add test for "pour" when target is in opposite split
• Add test for "take" on a target within a notebook cell when the notebook is in the opposite split. Note that this test prob won't work on legacy VSCode (<1.68.0)
• Add test for "paste after" when target is in opposite split. There is some slight trickiness around tracking which editor needs cursors updated (ie the one where paste happened), but then the original editor should be focused in the end

What to check for each test

We need to check the following:

• Ensure that the correct editor gets / keeps focus (as the case may be). Ie "paste" should keep focus, "pour should move it.
• Check that content / selections in all visible editors are correct

Notes

Could potentially re-use some stuff from crossCellsSetSelection

Questions

Should we support recording tests with multiple splits?