Docx reader tests: Introduce NoNormPandoc type.

This is just a wrapper around Pandoc that doesn't normalize with
`toString`. We want to make sure that our own normalization process
works. If, in the future, we are able to hook into the builder's
normalization, this will be removed.
Merge pull request #1361 from jkr/testNormalize

Docx reader tests: Introduce NoNormPandoc type.