Add a rustdoc test

This is just a smoke test which verifies that the expected files are
generated. Also makes the rmake tests have the right deps.
rustdoc: thread through the context for rendering

This partially reverts 8b5a317.
auto merge of #11032 : cmr/rust/rustdoc_test, r=alexcrichton

This is just a smoke test which verifies that the expected files are
generated.