Added ui tests, Limited serde(tag = "...") to structs with named field

Added ui test struct-representation/internally-tagged-unit
Added ui test struct-representation/internally-tagged-tuple
    
Limited the serde(tag = "...") to enums and structs with named field

test_suite/tests/ui/struct-representation/internally-taged-tuple.rs

@@ -0,0 +1,11 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Serialize)]
+#[serde(tag = "type")]
+struct S (
+    u8,
+    u8
+);
+
+fn main() {}

test_suite/tests/ui/struct-representation/internally-taged-tuple.stderr

@@ -0,0 +1,12 @@
+error: #[serde(tag = "...")] can only be used on enums and structs with named fields
+ --> $DIR/internally-taged-tuple.rs:6:10
+  |
+6 |   struct S (
+  |  __________^
+7 | |     u8,
+8 | |     u8
+9 | | );
+  | |_^
+
+error: aborting due to previous error
+

test_suite/tests/ui/struct-representation/internally-tagged-unit.rs

@@ -0,0 +1,8 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Serialize)]
+#[serde(tag = "type")]
+struct U;
+
+fn main() {}

test_suite/tests/ui/struct-representation/internally-tagged-unit.stderr

@@ -0,0 +1,8 @@
+error: #[serde(tag = "...")] can only be used on enums and structs with named fields
+ --> $DIR/internally-tagged-unit.rs:4:10
+  |
+4 | #[derive(Serialize)]
+  |          ^^^^^^^^^
+
+error: aborting due to previous error
+