Improve UI test coverage

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

test_suite/tests/ui/enum-representation/content-no-tag.rs

@@ -0,0 +1,11 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Serialize)]
+#[serde(content = "c")]
+enum E {
+    A(u8),
+    B(String),
+}
+
+fn main() {}

test_suite/tests/ui/enum-representation/content-no-tag.stderr

@@ -0,0 +1,8 @@
+error: #[serde(tag = "...", content = "...")] must be used together
+ --> $DIR/content-no-tag.rs:5:9
+  |
+5 | #[serde(content = "c")]
+  |         ^^^^^^^
+
+error: aborting due to previous error
+

test_suite/tests/ui/enum-representation/untagged-and-adjacent.rs

@@ -0,0 +1,12 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Serialize)]
+#[serde(untagged)]
+#[serde(tag = "t", content = "c")]
+enum E {
+    A(u8),
+    B(String),
+}
+
+fn main() {}

test_suite/tests/ui/enum-representation/untagged-and-adjacent.stderr

@@ -0,0 +1,20 @@
+error: untagged enum cannot have #[serde(tag = "...", content = "...")]
+ --> $DIR/untagged-and-adjacent.rs:5:9
+  |
+5 | #[serde(untagged)]
+  |         ^^^^^^^^
+
+error: untagged enum cannot have #[serde(tag = "...", content = "...")]
+ --> $DIR/untagged-and-adjacent.rs:6:9
+  |
+6 | #[serde(tag = "t", content = "c")]
+  |         ^^^
+
+error: untagged enum cannot have #[serde(tag = "...", content = "...")]
+ --> $DIR/untagged-and-adjacent.rs:6:20
+  |
+6 | #[serde(tag = "t", content = "c")]
+  |                    ^^^^^^^
+
+error: aborting due to 3 previous errors
+

test_suite/tests/ui/enum-representation/untagged-and-content.rs

@@ -0,0 +1,12 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Serialize)]
+#[serde(untagged)]
+#[serde(content = "c")]
+enum E {
+    A(u8),
+    B(String),
+}
+
+fn main() {}

test_suite/tests/ui/enum-representation/untagged-and-content.stderr

@@ -0,0 +1,14 @@
+error: untagged enum cannot have #[serde(content = "...")]
+ --> $DIR/untagged-and-content.rs:5:9
+  |
+5 | #[serde(untagged)]
+  |         ^^^^^^^^
+
+error: untagged enum cannot have #[serde(content = "...")]
+ --> $DIR/untagged-and-content.rs:6:9
+  |
+6 | #[serde(content = "c")]
+  |         ^^^^^^^
+
+error: aborting due to 2 previous errors
+