fix test_monotone_constraints often fails on MPI builds (#3683)
fix test_monotone_constraints often fails on MPI builds (#3683)

* Fix monotone constraint bug where split does not fulfill constraints.

* Fix indent.
microsoft/LightGBM,microsoft/LightGBM,henry0312/LightGBM,henry0312/LightGBM,henry0312/LightGBM,microsoft/LightGBM,microsoft/LightGBM,microsoft/LightGBM,henry0312/LightGBM,henry0312/LightGBM