get array out of ObservableArray for test, see https://github.com/phetsims/energy-forms-and-changes/issues/372
get array out of ObservableArray for test, see https://github.com/phetsims/energy-forms-and-changes/issues/372

(cherry picked from commit 3d784c6b3409f7a56e15877e538398a706b87fbf)

phetsims/energy-forms-and-changes,phetsims/energy-forms-and-changes,phetsims/energy-forms-and-changes