[SPARK-3786] [PySpark] speedup tests

This patch try to speed up tests of PySpark, re-use the SparkContext in tests.py and mllib/tests.py to reduce the overhead of create SparkContext, remove some test cases, which did not make sense. It also improve the performance of some cases, such as MergerTests and SortTests.


it's a stress test, will be done in SparkPerf.