test(parallel): Add regression test for fn_sig cycle arity ICE #153391

Author: TKanX

tests/ui/parallel-rustc/fn-sig-cycle-ice-153391.rs

@@ -0,0 +1,15 @@
+// Regression test for #153391.
+//
+//@ edition:2024
+//@ compile-flags: -Z threads=16
+//@ error-pattern: expected a type, found a trait
+
+trait A {
+    fn g() -> B;
+}
+
+trait B {
+    fn bar(&self, x: &A);
+}
+
+fn main() {}