[2p1cni][test][box] Update ref solution

* the test failed compared to the ref solution generated on the old
  master
* the reason for this is the change of how rho in context of gravity
  is calculated

* using rho = 0.5*(elemVolVars[scvf.insideScvIdx()].density(phaseIdx)
            + elemVolVars[scvf.outsideScvIdx()].density(phaseIdx));

  instead of interpolating the value of rho onto the face made the test
  pass w.r.t the the old solution

* we prefer interpolationg rho, however, therefore the ref solution is
  updated
* using the interpolated rho also decreases the number of timesteps from 8 to 7
  (compared to the old master)