Add tests against weird provided/required method behaviour

In `test/rustdoc/manual_impl.rs` there are now three structs:

* S1 implements and documents required method `a_method`.
* S2 implements and documents `a_method` as well as provided
  method `b_method`.
* S3 implements `a_method` and `b_method`, but only documents
  `b_method`.

For a struct, we want the rendered trait impls to include documentation
if and only if it appears on the trait implementation itself
(since users can just go to the trait definition for anything not
covered in the impl docs). This means we expect:

* S1, S2, and S3 to all include top-level trait impl docs.
* S1, S2, and S3 to exclude all trait definition docs.
* S1 to show impl docs for `a_method`.
* S2 to show impl docs for `a_method` and `b_method`.
* S3 to show impl docs for `b_method`.

These tests cover those cases.

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