Add (a lot of) tests to rustdoc

[GuillaumeGomez]

Since we now have a more complete way to compare two HTML contents, I think we can now strongly reinforce rustdoc's tests (which are a bit weak and lacking for now).

cc @rust-lang/docs

[QuietMisdreavus]

Some ideas:

• src/librustdoc/html/markdown.rs already has a #[cfg(test)] block, but it doesn't test the "hide lines of code with # " behavior.
• src/librustdoc/test.rs could use some tests to verify how it adds attributes to the test, whether it wraps tests in a fn main(), things like that.

[QuietMisdreavus]

One thing i'd like to see (that will probably need cooperation from some rustbuild compiletest experts) is the ability to test the stdout-printing behavior of rustdoc in various situations. We don't have UI tests for rustdoc the same way the compiler does, so right now there's no place for these in the testing setup.

[camelid]

ping: Is this still an issue?

[GuillaumeGomez]

Still waiting on #70533.