[SPARK-4887][MLlib] Fix a bad unittest in LogisticRegressionSuite

The original test doesn't make sense since if you step in, the lossSum is already NaN, 
and the coefficients are diverging. That's because the step size is too large for SGD, 
so it doesn't work. 

The correct behavior is that you should get smaller coefficients than the one 
without regularization. Comparing the values using 20000.0 relative error doesn't 
make sense as well.