[SPARK-21801][SPARKR][TEST] unit test randomly fail with randomforest

## What changes were proposed in this pull request?

fix the random seed to eliminate variability

## How was this patch tested?

jenkins, appveyor, lots more jenkins

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #19018 from felixcheung/rrftest.