Improve UI test coverage

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

test_suite/tests/ui/unsupported/union_de.rs

@@ -0,0 +1,10 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Deserialize)]
+union Union {
+    x: u8,
+    y: (),
+}
+
+fn main() {}

test_suite/tests/ui/unsupported/union_de.stderr

@@ -0,0 +1,11 @@
+error: Serde does not support derive for unions
+ --> $DIR/union_de.rs:5:1
+  |
+5 | / union Union {
+6 | |     x: u8,
+7 | |     y: (),
+8 | | }
+  | |_^
+
+error: aborting due to previous error
+

test_suite/tests/ui/unsupported/union_ser.rs

@@ -0,0 +1,10 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Deserialize)]
+union Union {
+    x: u8,
+    y: (),
+}
+
+fn main() {}

test_suite/tests/ui/unsupported/union_ser.stderr

@@ -0,0 +1,11 @@
+error: Serde does not support derive for unions
+ --> $DIR/union_ser.rs:5:1
+  |
+5 | / union Union {
+6 | |     x: u8,
+7 | |     y: (),
+8 | | }
+  | |_^
+
+error: aborting due to previous error
+