Test: transition into multi-root workspace without window reload

[bpasero]

Refs: #32945

• Win @gregvanl
• Mac @weinand
• Linux @isidorn

Complexity: 4

We no longer require a window reload when entering a workspace.

Verify for the following actions:

• Empty window: Add folders...
• Empty window: Save workspace as...
• 1-folder window: Add folders...
• 1-folder window: Save workspace as...
• Workspace window: Save workspace as...

That:

• our UI updates accordingly (e.g. explorer, window title)
• our migration of UI state (local storage) and hot-exit backups works properly
  • no data-loss: your dirty files are kept and restored when you now do a restart and new dirty files that you create after the transition are also restored properly
  • your UI state is kept and restored when you now restart the instance
• any window-workspace settings are migrated over to the workspace file (only when transitioning from a 1-folder window)
• extensions work properly still (we do restart the extension host!)

[bpasero]

/cc @sandy081

[isidorn]

Works well 👍