test(onboarding): lose the canvas inside the preview, not during setup

`beginTour` awaits the renderer switch before `delay(INTRO_PREVIEW_MS)`, so
flipping `linearMode` immediately after calling it changed the value while
setup was still running rather than during the preview the test names.

The test was already mutation-proof either way - removing the post-delay
re-check fails it and nothing else - because no check sits between the
setting write and the delay. But that is a property of today's code, not of
the test, so a later check added there would have quietly made this pass for
the wrong reason.

Flushes setup with a zero-length tick first.

Addresses #14673 (comment)

Author: petros-double-test1

src/renderer/extensions/firstRunTour/tour/useFirstRunTourController.test.ts

@@ -374,6 +374,10 @@ describe('useFirstRunTourController', () => {
       const controller = await freshController()
 
       const starting = controller.beginTour('image_z_image_turbo')
+      // Flush the renderer switch first, so the canvas is lost inside the
+      // preview delay rather than while beginTour is still setting up. Only
+      // the post-delay re-check can catch it from there.
+      await vi.advanceTimersByTimeAsync(0)
       mocks.linearMode.value = true
       await vi.advanceTimersByTimeAsync(INTRO_PREVIEW_MS)