BigQuery: 'test_bigquery_magic_w_maximum_bytes_billed_invalid' test uses real client.

[tseaver]

The unit test fails if there are no valid credentials in the environment:

______________ test_bigquery_magic_w_maximum_bytes_billed_invalid ______________

    @pytest.mark.usefixtures("ipython_interactive")
    def test_bigquery_magic_w_maximum_bytes_billed_invalid():
        ip = IPython.get_ipython()
        ip.extension_manager.load_extension("google.cloud.bigquery")
        magics.context._project = None
    
        sql = "SELECT 17 AS num"
    
        with pytest.raises(ValueError):
>           ip.run_cell_magic("bigquery", "--maximum_bytes_billed=abc", sql)

tests/unit/test_magics.py:565: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
.nox/unit-3-6/lib/python3.6/site-packages/IPython/core/interactiveshell.py:2358: in run_cell_magic
    result = fn(*args, **kwargs)
google/cloud/bigquery/magics.py:400: in _cell_magic
    default_query_job_config=context.default_query_job_config,
google/cloud/bigquery/client.py:167: in __init__
    project=project, credentials=credentials, _http=_http
../core/google/cloud/client.py:226: in __init__
    _ClientProjectMixin.__init__(self, project=project)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <google.cloud.bigquery.client.Client object at 0x7f4bb6739d68>
project = None

    def __init__(self, project=None):
        project = self._determine_default(project)
        if project is None:
            raise EnvironmentError(
>               "Project was not passed and could not be "
                "determined from the environment."
            )
E           OSError: Project was not passed and could not be determined from the environment.

We want unit tests to run in complete isolation from any possibility of triggering real API requests. This test actually expects to run a real query, which means it is a system test.