[SPARK-16307][ML] Add test to verify the predicted variances of a DT on toy data

## What changes were proposed in this pull request?

The current tests assumes that `impurity.calculate()` returns the variance correctly. It should be better to make the tests independent of this assumption. In other words verify that the variance computed equals the variance computed manually on a small tree.

## How was this patch tested?

The patch is a test....

Author: MechCoder <mks542@nyu.edu>

Closes #13981 from MechCoder/dt_variance.