Add a test for unused const parameters

Author: varkor

src/test/ui/const-generics/unused-const-param.rs

@@ -0,0 +1,8 @@
+// run-pass
+
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
+
+struct A<const N: usize>; // ok
+
+fn main() {}

src/test/ui/const-generics/unused-const-param.stderr

@@ -0,0 +1,6 @@
+warning: the feature `const_generics` is incomplete and may cause the compiler to crash
+  --> $DIR/unused-const-param.rs:3:12
+   |
+LL | #![feature(const_generics)]
+   |            ^^^^^^^^^^^^^^
+