'Verify that method returns given provider list as comma separated string.'

@ddt.data(None, ['asu'], ['asu', 'co'], ['asu', 'co', 'mit'])
def test_make_providers_strings(self, providers):

    provider_string = make_providers_strings(providers)
    if (not providers):
        self.assertEqual(provider_string, None)
    elif (len(providers) == 1):
        self.assertEqual(provider_string, providers[0])
    elif (len(providers) == 2):
        self.assertEqual(provider_string, 'asu    and    co')
    else:
        self.assertEqual(provider_string, 'asu,    co,    and    mit')
