Modify how to disable logging in unit test.

Modify how to disable logging in unit test.

Logs are only disabled when the unit tests are run from cmdline.
It allows running them via pytest_suite_runner.py and printing
their results to functest.log.

You should notice that the unit test must disable the logger of the
tested TestCase (e.g. testcase.TestCase.logger.disabled = 1). Otherwise
false logs will be printed in console.

Change-Id: Ic171e4575c8bab2e52db1897ae026b61a03b6247
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>