Rewrite a subscription test

Rewrite method `test_negative_create_1` in module
`tests.foreman.api.test_subscription`. The test docstring states that it should
not be possible to upload the same manifest to two organizations, but the actual
test code did not verify this assertion. Make that happen.

    $ nosetests tests/foreman/api/test_subscription.py -m test_negative_create_1
    .
    ----------------------------------------------------------------------
    Ran 1 test in 31.106s

    OK