Improve UI test coverage

With this commit I believe I've covered all `compile_error!`-based
errors.

test_suite/tests/ui/default-attribute/enum_path.rs

@@ -0,0 +1,8 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Deserialize)]
+#[serde(default = "default_e")]
+enum E {
+    S { f: u8 },
+}

test_suite/tests/ui/default-attribute/enum_path.stderr

@@ -0,0 +1,8 @@
+error: #[serde(default = "...")] can only be used on structs with named fields
+ --> $DIR/enum_path.rs:6:1
+  |
+6 | enum E {
+  | ^^^^
+
+error: aborting due to previous error
+

test_suite/tests/ui/default-attribute/nameless_struct_fields_path.rs

@@ -0,0 +1,8 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Deserialize)]
+#[serde(default = "default_t")]
+struct T(u8, u8);
+
+fn main() {}

test_suite/tests/ui/default-attribute/nameless_struct_fields_path.stderr

@@ -0,0 +1,8 @@
+error: #[serde(default = "...")] can only be used on structs with named fields
+ --> $DIR/nameless_struct_fields_path.rs:6:9
+  |
+6 | struct T(u8, u8);
+  |         ^^^^^^^^
+
+error: aborting due to previous error
+