jsondocck: add `$FILE` built-in variable This built-in variable will allow accessing the full path to the currently tested file and allow to test things like source code spans generated by rustdoc-json, and that is exactly the reason why I've come up with the idea to add this [futher discussion on zulip](https://rust-lang.zulipchat.com/#narrow/stream/266220-t-rustdoc/topic/rustdoc-json.20test.20help/near/449039819) |
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