[SPARK-19604][TESTS] Log the start of every Python test

## What changes were proposed in this pull request?
Right now, we only have info level log after we finish the tests of a Python test file. We should also log the start of a test. So, if a test is hanging, we can tell which test file is running.

## How was this patch tested?
This is a change for python tests.

Author: Yin Huai <yhuai@databricks.com>

Closes #16935 from yhuai/SPARK-19604.

python/run-tests.py

@@ -72,7 +72,7 @@ def run_individual_python_test(test_name, pyspark_python):
         'PYSPARK_PYTHON': which(pyspark_python),
         'PYSPARK_DRIVER_PYTHON': which(pyspark_python)
     })
-    LOGGER.debug("Starting test(%s): %s", pyspark_python, test_name)
+    LOGGER.info("Starting test(%s): %s", pyspark_python, test_name)
     start_time = time.time()
     try:
         per_test_output = tempfile.TemporaryFile()