Add const_fn in generics test

src/test/ui/const-generics/min_const_generics/const_fn_in_generics.rs

@@ -0,0 +1,17 @@
+// run-pass
+
+#![feature(min_const_generics)]
+
+const fn identity<const T: u32>() -> u32 { T }
+
+#[derive(Eq, PartialEq, Debug)]
+pub struct ConstU32<const U: u32>;
+
+pub fn new() -> ConstU32<{ identity::<3>() }> {
+  ConstU32::<{ identity::<3>() }>
+}
+
+fn main() {
+  let v = new();
+  assert_eq!(v, ConstU32::<3>);
+}