Export the solution environment convert's own example uses

[peterbjohnson]

in2lambda convert in2lambda/filters/PartsOneSol/example.tex PartsOneSol returns both questions with no worked solution. The PartsOneSol filter matches a Div only when pf.stringify(elem.content[0].content) == "Solution"; pandoc writes the example's solution environment as a Div with class solution and no such text, so the filter never sees a solution. t49 found this by comparing the draft route with convert on the package's own example.

Make the filter recognise the solution environment as pandoc writes it: a Div whose classes hold solution, and keep the text match for documents that write a Solution heading inside the div. Check the other three filters for the same test and fix them the same way. A test in tests/ runs each filter's example.tex through runner and asserts every question that has a solution environment in the source has a non-empty worked solution in the set. Done when the four examples export their solutions and the existing filter tests pass.

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Workbench ticket t52.