In a script 'python/run-tests', log file name is represented by a variable 'LOG_FILE' and it is used in run-tests. But, there are some hard-coded log file name in the script.

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #2905 from sarutak/SPARK-4058 and squashes the following commits:

7710490 [Kousuke Saruta] Fixed python/run-tests not to use hard-coded log file name