Test: centered editor layout

[isidorn]

Refs: #15684

• any os: @sandy081
• any os: @RMacfarlane

Complexity: 3

This milestone we have added centered editor layout. Verify:

• You can enter to this layout via command palette, view menu, by entering zen mode
• You can use the vertical sashes to control the size of the editor
• Sashes are paired unless you hold the alt key while dragging them
• Editors are automatically not centered if more than one is shown side-by-side (also for diff viewer)
• If tabs are hidden the editor title is also centered
• Double click on sash returns them to default golden ratio
• Changing sash size and centered mode are preserved across vscode sessions
• Setting zenMode.centerLayout behaves as expected