Improve UI test coverage

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

test_suite/tests/ui/malformed/bound.rs

@@ -0,0 +1,10 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Serialize)]
+struct S {
+    #[serde(bound(unknown))]
+    x: (),
+}
+
+fn main() {}

test_suite/tests/ui/malformed/bound.stderr

@@ -0,0 +1,8 @@
+error: malformed bound attribute, expected `bound(serialize = ..., deserialize = ...)`
+ --> $DIR/bound.rs:6:19
+  |
+6 |     #[serde(bound(unknown))]
+  |                   ^^^^^^^
+
+error: aborting due to previous error
+

test_suite/tests/ui/malformed/rename.rs

@@ -0,0 +1,10 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Serialize)]
+struct S {
+    #[serde(rename(unknown))]
+    x: (),
+}
+
+fn main() {}

test_suite/tests/ui/malformed/rename.stderr

@@ -0,0 +1,8 @@
+error: malformed rename attribute, expected `rename(serialize = ..., deserialize = ...)`
+ --> $DIR/rename.rs:6:20
+  |
+6 |     #[serde(rename(unknown))]
+  |                    ^^^^^^^
+
+error: aborting due to previous error
+