Improve UI test coverage

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

test_suite/tests/ui/precondition/serialize_field_identifier.rs

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

test_suite/tests/ui/precondition/serialize_field_identifier.stderr

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

test_suite/tests/ui/precondition/serialize_variant_identifier.rs

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

test_suite/tests/ui/precondition/serialize_variant_identifier.stderr

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