tests: don't use tmp as the default log dir with run.py

tests: don't use tmp as the default log dir with run.py

The log file directory is configurable with run.py using the
--log-dir argument. This patch removes the use of /tmp as
the default dir for storing all test logs. The default
log dir is now set to show the year, month and day
of the test run. This provides a more meaningful aggregation
of test logs for effective troubleshooting. The default log
dir is set to <CWD>/test-run-YYYY-MM-DD.

Type: improvement
Change-Id: I6c9002e961f6e06fc953ca42d86febf4f218e566
Signed-off-by: Naveen Joy <najoy@cisco.com>