Tests for rustdoc outputs

[lifthrasiir]

We already have a heap of various rustdoc bugs, most of them are reported and verified by hand. We need a good facility to test rustdoc outputs with the expected output. For example, the "expected output" might be a set of expected files and expected set of strings to appear from each file.

A superset of #13444.

[tomjakubowski]

There are a few handwritten tests like this in run-make, like https://github.com/rust-lang/rust/tree/master/src/test/run-make/rustdoc-where and https://github.com/rust-lang/rust/tree/master/src/test/run-make/rustdoc-search-index but good coverage isn't there and it's somewhat awkward writing them.

[tomjakubowski]

Since most rustdoc bugs seem to be in clean and clean::inline, it might be worth adding some unit tests for those at least. It could be tricky to produce fixture data for inlined docs though, which tend to cause the most headaches.

[alexcrichton]

With htmldockck this is basically done. Awesome work again @lifthrasiir!