Create functional test to check slicing functionality

During #930 , issues became apparent when data at a cycling frequency of P3M using monthly slicing in the request file caused the final time point to be removed from the cube and hence trigger a time contiguity QC error.

This has since been resolved by changes to `new_varaible.py` in #930. A functional test should be created to ensure that this bug does not get re introduced by future changes.

Read new_varaible.py and the changes from #930 first, then trace how a request file applies monthly slicing to P3M data. Add a functional regression test covering the affected case; done means the final time point remains in the cube and the time-contiguity QC error is not triggered.

Written by the indexing model from the issue text.