Some htmldocck tests could be cleaned up

For example, the `rustdoc-where` tests originally used regexes as a kludge to avoid trying to match HTML tags, but with `htmldocck` they can now check against text nodes. 

Something like:

```
// @matches foo/struct.Alpha.html '//pre' "Alpha.*where.*A:.*MyTrait"
```

can be greatly clarified like:

```
// @matches foo/struct.Alpha.html '//pre' "pub struct Alpha<A> where A: MyTrait"
```
