Add tests for different column naming conventions

[Gordon Watts]

We needed these for Awkward, but it should be there for everything:

• with a list of column names
• With a single name
• With nothing (and dict in the last select statement).

As an example:

def test_sx_xaod_awkward_no_columns(async_mock):
    'Test a request for awkward arrays from an xAOD backend'
    sx = async_mock(spec=ServiceXDataset)
    sx.first_supported_datatype.return_value = 'root'
    ds = ServiceXSourceXAOD(sx)
    q = ds.Select("lambda e: e.MET").AsAwkwardArray()

    q.value()

[Gordon Watts]

This is covered by some tests from other sources, I think. We'll close this unless we find a problem.