[rustdoc-json] Output is currently under-tested

[CraftSpider]

Currently, the output of rustdoc's JSON backend has very few tests, which only assure some of the most simple functionality. It would be nice to at least have tests for basic forms of each item, as well as any known issues. A quick list of things not really tested (non-exhaustive):

• Impls
• Associated Items
• Consts/Statics
• Extern items (all forms)
• Macros
• Types

It also would be nice to know the desired layout of the tests. Currently, the directory holds the form

root
| structs
- | plain
  | tuple
  | generic
| traits
- | has_body
| unions
- | plain
| ...

This is slightly different from how normal rustdoc ui tests are structured, but on the other hand HTML output is very different and serves a somewhat different goal. If we want to establish guidelines on the test directory layout, now is a good time to discuss it, before the directories fill up.