Migrate a test to use @snapshot

I'd been thinking about implementing snapshot testing for a while, but
This test is what finally made me do it. It really benefits from using
snapshot testing, so it's a good initial place to use `@snapshot`.

Author: camelid

src/test/rustdoc/mixing-doc-comments-and-attrs.S1_top-doc.html

@@ -0,0 +1,4 @@
+<div class="docblock"><p>Hello world!
+Goodbye!
+Hello again!</p>
+</div>

src/test/rustdoc/mixing-doc-comments-and-attrs.S2_top-doc.html

@@ -0,0 +1,4 @@
+<div class="docblock"><p>Hello world!</p>
+<p>Goodbye!
+Hello again!</p>
+</div>

src/test/rustdoc/mixing-doc-comments-and-attrs.rs

@@ -1,23 +1,15 @@
 #![crate_name = "foo"]
 
 // @has 'foo/struct.S1.html'
-// @count - '//details[@class="rustdoc-toggle top-doc"]/div[@class="docblock"]/p' \
-//     1
-// @has - '//details[@class="rustdoc-toggle top-doc"]/div[@class="docblock"]/p[1]' \
-//     'Hello world! Goodbye! Hello again!'
+// @snapshot S1_top-doc - '//details[@class="rustdoc-toggle top-doc"]/div[@class="docblock"]'
 
 #[doc = "Hello world!\n\n"]
 /// Goodbye!
 #[doc = "  Hello again!\n"]
 pub struct S1;
 
 // @has 'foo/struct.S2.html'
-// @count - '//details[@class="rustdoc-toggle top-doc"]/div[@class="docblock"]/p' \
-//     2
-// @has - '//details[@class="rustdoc-toggle top-doc"]/div[@class="docblock"]/p[1]' \
-//     'Hello world!'
-// @has - '//details[@class="rustdoc-toggle top-doc"]/div[@class="docblock"]/p[2]' \
-//     'Goodbye! Hello again!'
+// @snapshot S2_top-doc - '//details[@class="rustdoc-toggle top-doc"]/div[@class="docblock"]'
 
 /// Hello world!
 ///