Improve UI test coverage

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

test_suite/tests/ui/rename/container_unknown_rename_rule.rs

@@ -0,0 +1,12 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Serialize)]
+#[serde(rename_all = "abc")]
+struct S {
+    name: u8,
+    long_name: u8,
+    very_long_name: u8,
+}
+
+fn main() {}

test_suite/tests/ui/rename/container_unknown_rename_rule.stderr

@@ -0,0 +1,8 @@
+error: unknown rename rule for #[serde(rename_all = "abc")]
+ --> $DIR/container_unknown_rename_rule.rs:5:22
+  |
+5 | #[serde(rename_all = "abc")]
+  |                      ^^^^^
+
+error: aborting due to previous error
+

test_suite/tests/ui/rename/variant_unknown_rename_rule.rs

@@ -0,0 +1,14 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Serialize)]
+enum S {
+    #[serde(rename_all = "abc")]
+    V {
+        name: u8,
+        long_name: u8,
+        very_long_name: u8,
+    },
+}
+
+fn main() {}

test_suite/tests/ui/rename/variant_unknown_rename_rule.stderr

@@ -0,0 +1,8 @@
+error: unknown rename rule for #[serde(rename_all = "abc")]
+ --> $DIR/variant_unknown_rename_rule.rs:6:26
+  |
+6 |     #[serde(rename_all = "abc")]
+  |                          ^^^^^
+
+error: aborting due to previous error
+