Add regression test for private fields in tuple struct

tests/rustdoc/private-fields-tuple-struct.rs

@@ -0,0 +1,15 @@
+// This test checks the diplay of "/* private fields */" sentence in tuple structs.
+#![crate_name = "foo"]
+
+// @has 'foo/struct.A.html' '//*[@class="rust item-decl"]/code' 'pub struct A(pub u8, _);'
+pub struct A(pub u8, u8);
+// @has 'foo/struct.B.html' '//*[@class="rust item-decl"]/code' 'pub struct B(_, pub u8);'
+pub struct B(u8, pub u8);
+// @has 'foo/struct.C.html' '//*[@class="rust item-decl"]/code' 'pub struct C(_, pub u8, _);'
+pub struct C(u8, pub u8, u8);
+// @has 'foo/struct.D.html' '//*[@class="rust item-decl"]/code' 'pub struct D(pub u8, _, pub u8);'
+pub struct D(pub u8, u8, pub u8);
+// @has 'foo/struct.E.html' '//*[@class="rust item-decl"]/code' 'pub struct E(/* private fields */);'
+pub struct E(u8);
+// @has 'foo/struct.F.html' '//*[@class="rust item-decl"]/code' 'pub struct F(/* private fields */);'
+pub struct F(u8, u8);