test: demonstrate PredicateBoxExt requires Sized

This test should fail, and the next commit should make it pass.

src/boxed.rs

@@ -115,3 +115,14 @@ where
 }
 
 impl<P, Item> PredicateBoxExt<Item> for P where P: Predicate<Item> {}
+
+#[cfg(test)]
+mod test {
+    use crate::prelude::*;
+
+    #[test]
+    fn unsized_boxed() {
+        let p = predicate::always().boxed();
+        p.eval("4");
+    }
+}