Add test for tuple struct documentation fields

Author: GuillaumeGomez

src/test/rustdoc/tuple-struct-fields-doc.rs

@@ -0,0 +1,19 @@
+#![crate_name = "foo"]
+
+// @has foo/struct.Foo.html
+// @has - '//h2[@id="fields"]' 'Tuple Fields'
+// @has - '//h3[@class="sidebar-title"]/a[@href="#fields"]' 'Tuple Fields'
+// @has - '//*[@id="structfield.0"]' '0: u32'
+// @has - '//*[@id="main"]/div[@class="docblock"]' 'hello'
+// @!has - '//*[@id="structfield.1"]'
+// @has - '//*[@id="structfield.2"]' '2: char'
+// @has - '//*[@id="structfield.3"]' '3: i8'
+// @has - '//*[@id="main"]/div[@class="docblock"]' 'not hello'
+pub struct Foo(
+    /// hello
+    pub u32,
+    char,
+    pub char,
+    /// not hello
+    pub i8,
+);