Create unit test for StructuredDataset.variable_time_axis

In cdds/qc/dataset/dataset.py, some changes to facilitate CMIP7 production were made to the function `variable_time_axis` to allow bounds to be grabbed from the time variable `climatology_bnds` (required in CMIP7 for some datasets). A unittest should be written to test this function and ensure the ability to get bounds from `climatology_bnds` is maintained.

For further clarity see PR #936

Start in cdds/qc/dataset/dataset.py by reading variable_time_axis, then review PR #936 for the CMIP7-related change. Add a unittest covering retrieval of bounds from climatology_bnds and run the relevant dataset tests to confirm this behavior remains covered.

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