Test: Master-detail SCM

[joaomoreno]

#33363

• Windows - @chrmarti
• Linux - @bpasero
• macOS - @jrieken

Complexity: 4

The SCM viewlet has changed how it behaves wrt multiple scm repositories:

• When a single repository is open, everything should work as before
• When multiple repositories are open, one should be visible by default
  • You can multi-select in the list to have multiple splitviews in the scm viewlet
  • Make sure actions work
  • Make sure interactions make sense
• Make sure other features around SCM are unbroken