By Kevin Robinson-Avila / Journal Staff Writer   Virgin Galactic’s VSS Unity in 2021. Company leaders told investors Thursday that the firm remains on track to restart test flights of its existing spaceships by January, and then begin flying paying passengers to space next spring. (Roberto E. Rosales/Journal) Virgin Galactic remains on track to restart test flights of its existing spaceships by January, and then begin flying paying passengers to space next spring, the company told investors in a third-quarter earnings call Thursday. Assuming there are no new setbacks, that means an imminent end to a nearly 18-month flight hiatus,...