Initial MSW tests for RTE/PTE

Asherah Connorrequested to merge
kiv/initial-msw-tests-for-rte-pte into master

What does this MR do and why?

Introduces MSW test machinery and pilot tests for the RTE/PTE.

We have a fixture generator which posts to the Markdown preview endpoint, the preview MSW handler which draws from those fixtures, assorted helpers, and a couple examples to get us started.

We also refactor out mountWorkItemCreatePage so we can use it too.

Part of Rewrite PTE/RTE tests using MSW (#630056).

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