Fix test

Author: odersky

tests/neg-custom-args/captures/i21401.check

@@ -8,3 +8,7 @@
    |                                                          ^^^^^^^^^^^^^^^^^^^
    |              The expression's type Res is not allowed to capture the root capability `cap` in its part box IO^.
    |              This usually means that a capability persists longer than its allowed lifetime.
+-- Error: tests/neg-custom-args/captures/i21401.scala:18:21 ------------------------------------------------------------
+18 |  val y: IO^{x*} = x.unbox // error
+   |                   ^^^^^^^
+   |                   Local reach capability x* leaks into capture scope of method test2

tests/neg-custom-args/captures/i21401.scala

@@ -15,5 +15,5 @@ def test2() =
   val a = usingIO[IO^](x => x) // error: The expression's type IO^ is not allowed to capture the root capability `cap`
   val leaked: [R, X <: Boxed[IO^] -> R] -> (op: X) -> R = usingIO[Res](mkRes) // error: The expression's type Res is not allowed to capture the root capability `cap` in its part box IO^
   val x: Boxed[IO^] = leaked[Boxed[IO^], Boxed[IO^] -> Boxed[IO^]](x => x)
-  val y: IO^{x*} = x.unbox
+  val y: IO^{x*} = x.unbox // error
   y.println("boom")