rfctr(docx): move docx unit tests to bottom (#3011)
No code changes, strictly this single block move.
Move `Describe_DocxPartitioner` unit-test class to bottom so
`DescribeDocxPartitionerOptions` unit-test to follow in subsequent
commit will be together with it. Integration tests first, then unit
tests, for consistency with other test modules e.g. test_pptx.
I added `Describe_DocxPartitioner` soon after I arrived, before we
adopted the convention of placing unit-tests after integration tests.
Move this so we can maintain that consistency with the block of tests to
follow in a closely subsequent PR.
