test(dirvish): cover cj/get-project-root with stubbed APIs

`cj/get-project-root' was already in the right shape (pure dispatch over `projectile-project-root' and `project-current'), but had no tests.  Add four covering the four meaningful states: projectile finds a root, projectile returns nil and project.el finds one, projectile errors and the wrapper falls through to project.el, and neither finds anything.  The tests stub all three external symbols (`projectile-project-root', `project-current', `project-root') so they exercise the function without a project on disk.

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