lsifstore: add API docs tests

Created by: slimsag

After this change, we have decent test coverage for:

• WriteDocumentationSearchPrework
• WriteDocumentationPages
• WriteDocumentationSearch
• WriteDocumentationPathInfo
• WriteDocumentationMappings
• documentationPathIDToID
• documentationPathIDToFilePath
• DocumentationPage
• DocumentationPathInfo
• DocumentationDefinitions

It's not perfect - more could be done to improve these, make them more legible, etc. But it's a start, and decent coverage.

Helps #20567

Helps #21938

Signed-off-by: Stephen Gutekanst [email protected]