WinHelp, SHG, MRB: add tests

Currently, Oddities.WinHelp completely lacks any tests. We should add them.

It's quite hard to find any test data for the library, though. I'm not aware of any modern tooling that would allow creating a `.hlp` file, and also I'm not aware of any free `.hlp` file samples (that we could commit into the repository to use as test data).

We'll have to find some.