Running run-tests.py with Python 2.6 cause following error:
```
Running PySpark tests. Output is in python//Users/tomohiko/.jenkins/jobs/pyspark_test/workspace/python/unit-tests.log
Will test against the following Python executables: ['python2.6', 'python3.4', 'pypy']
Will test the following Python modules: ['pyspark-core', 'pyspark-ml', 'pyspark-mllib', 'pyspark-sql', 'pyspark-streaming']
Traceback (most recent call last):
File "./python/run-tests.py", line 196, in <module>
main()
File "./python/run-tests.py", line 159, in main
python_implementation = subprocess.check_output(
AttributeError: 'module' object has no attribute 'check_output'
...
```
The cause of this error is using subprocess.check_output function, which exists since Python 2.7.
(ref. https://docs.python.org/2.7/library/subprocess.html#subprocess.check_output)
Author: cocoatomo <cocoatomo77@gmail.com>
Closes#7161 from cocoatomo/issues/8763-test-fails-py26 and squashes the following commits:
cf4f901 [cocoatomo] [SPARK-8763] backport process.check_output function from Python 2.7
fdcad6ef48