Add test for coroutine attribute

tests/ui/coroutine/invalid_attr_usage.rs

@@ -0,0 +1,11 @@
+//! The `coroutine` attribute is only allowed on closures.
+
+#![feature(coroutines)]
+
+#[coroutine]
+//~^ ERROR: attribute should be applied to closures
+struct Foo;
+
+#[coroutine]
+//~^ ERROR: attribute should be applied to closures
+fn main() {}

tests/ui/coroutine/invalid_attr_usage.stderr

@@ -0,0 +1,14 @@
+error: attribute should be applied to closures
+  --> $DIR/invalid_attr_usage.rs:5:1
+   |
+LL | #[coroutine]
+   | ^^^^^^^^^^^^
+
+error: attribute should be applied to closures
+  --> $DIR/invalid_attr_usage.rs:9:1
+   |
+LL | #[coroutine]
+   | ^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+