Extraction of filter clause from SQL statements has bugs

**Describe the bug**
DVT creates the string that is used as a filter clause to create partitions. The extraction of this filter clause is hack - see explanation [here](https://github.com/GoogleCloudPlatform/professional-services-data-validator/blob/e15d8fd5bdf413d0d8ea3d43319c35c4a78607df/data_validation/partition_builder.py#L109).

The [unit test](https://github.com/GoogleCloudPlatform/professional-services-data-validator/blob/e15d8fd5bdf413d0d8ea3d43319c35c4a78607df/tests/unit/test_partition_builder.py#L653) contains some sample SQL where the _extract_where does not extract the filter clause correctly. These tests are currently skipped.

Most customers will not run into these issues - even if they do - the validation will fail due to malformed SQL rather than give incorrect results.