Improve UI test coverage

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

test_suite/tests/ui/identifier/other_untagged.rs

@@ -0,0 +1,11 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Deserialize)]
+#[serde(untagged)]
+enum F {
+    #[serde(other)]
+    Other,
+}
+
+fn main() {}

test_suite/tests/ui/identifier/other_untagged.stderr

@@ -0,0 +1,9 @@
+error: #[serde(other)] cannot appear on untagged enum
+ --> $DIR/other_untagged.rs:7:5
+  |
+7 | /     #[serde(other)]
+8 | |     Other,
+  | |_________^
+
+error: aborting due to previous error
+

test_suite/tests/ui/identifier/other_variant.rs

@@ -0,0 +1,11 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Deserialize)]
+#[serde(variant_identifier)]
+enum F {
+    #[serde(other)]
+    Other,
+}
+
+fn main() {}

test_suite/tests/ui/identifier/other_variant.stderr

@@ -0,0 +1,9 @@
+error: #[serde(other)] may not be used on a variant identifier
+ --> $DIR/other_variant.rs:7:5
+  |
+7 | /     #[serde(other)]
+8 | |     Other,
+  | |_________^
+
+error: aborting due to previous error
+

test_suite/tests/ui/identifier/serialize.rs

@@ -1,11 +0,0 @@
-#[macro_use]
-extern crate serde_derive;
-
-#[derive(Serialize)]
-#[serde(field_identifier)]
-enum F {
-    A,
-    B,
-}
-
-fn main() {}

test_suite/tests/ui/identifier/serialize.stderr

@@ -1,12 +0,0 @@
-error: field identifiers cannot be serialized
- --> $DIR/serialize.rs:5:1
-  |
-5 | / #[serde(field_identifier)]
-6 | | enum F {
-7 | |     A,
-8 | |     B,
-9 | | }
-  | |_^
-
-error: aborting due to previous error
-