Add test that caused query cycle with unevaluated const

tests/ui/traits/next-solver/unevaluated_const_query_cycle.rs

@@ -0,0 +1,20 @@
+//@ compile-flags: -Znext-solver
+//@ check-pass
+
+// Regression test for https://github.com/rust-lang/trait-system-refactor-initiative/issues/249
+
+const CONST: &str = "hi";
+
+trait ToUnit {
+    type Assoc;
+}
+impl<T> ToUnit for T {
+    type Assoc = ();
+}
+
+fn foo()
+where
+    <[u8; CONST.len()] as ToUnit>::Assoc: Sized,
+{}
+
+fn main(){}