add test for #97725

Fixes #97725

tests/ui/const-generics/occurs-check/unify-fixpoint.rs

@@ -1,3 +1,8 @@
+// -Zunstable-options added as test for ICE #97725 (left == right)`
+// left: `Binder(<[u8; _] as std::default::Default>, [])`,
+// right: `Binder(<[u8; 4] as std::default::Default>, [])
+
+//@ compile-flags: -Zunstable-options
 //@ check-pass
 #![feature(generic_const_exprs)] //~ WARN the feature `generic_const_exprs` is incomplete
 

tests/ui/const-generics/occurs-check/unify-fixpoint.stderr

@@ -1,5 +1,5 @@
 warning: the feature `generic_const_exprs` is incomplete and may not be safe to use and/or cause compiler crashes
-  --> $DIR/unify-fixpoint.rs:2:12
+  --> $DIR/unify-fixpoint.rs:7:12
    |
 LL | #![feature(generic_const_exprs)]
    |            ^^^^^^^^^^^^^^^^^^^