Add tests of #[serde(default)] attribute for units and unions

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

@@ -0,0 +1,7 @@
+error: #[serde(default)] can only be used on structs that have fields
+ --> tests/ui/default-attribute/unit.rs:3:10
+  |
+3 | #[derive(Deserialize)]
+  |          ^^^^^^^^^^^
+  |
+  = note: this error originates in the derive macro `Deserialize` (in Nightly builds, run with -Z macro-backtrace for more info)