Allow running unittest.TestCase via run_tests.py

Allow running unittest.TestCase via run_tests.py

pytest_suite_runner can run any unittest.TestCase thanks to name passed
as run arg. Here is the related functest/ci/testcases.yaml:

  run:
      module: 'functest.core.pytest_suite_runner'
      class: 'PyTestSuiteRunner'
      args:
          name: 'functest.tests.unit.odl.test_odl'

Change-Id: I7c2c611c4afbad1e52bfdca4a3288ccc21491389
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>