Extend test cases

Author: odersky

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

@@ -32,7 +32,14 @@
    |
    |                  where:    ^ refers to a fresh root capability classified as Mutable in the type of value x
 -- Error: tests/neg-custom-args/captures/mutability.scala:22:5 ---------------------------------------------------------
-22 |  r2.set(33) // error
+22 |  r1.set(33) // error
    |  ^^^^^^
-   |  cannot call update method set from (r2 : Ref[Int]),
-   |  since its capture set {r2} is read-only
+   |  cannot call update method set from (r1 : Ref[Int]),
+   |  since its capture set {r1} is read-only
+-- Error: tests/neg-custom-args/captures/mutability.scala:27:7 ---------------------------------------------------------
+27 |  r3.x.set(33) // error
+   |  ^^^^^^^^
+   |  cannot call update method set from (r3.x : Ref[Int]^),
+   |  since the capture set {r3} of its prefix (r3 : Ref2[Int]) is read-only
+   |
+   |  where:    ^ refers to a fresh root capability classified as Mutable in the type of value x

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

@@ -18,5 +18,12 @@ class Ref2[T](init: T) extends caps.Mutable:
 def test =
   val r = Ref(22)
   r.set(33) // ok
-  val r2: Ref[Int] = Ref(22)
-  r2.set(33) // error
+  val r1: Ref[Int] = Ref(22)
+  r1.set(33) // error
+
+  val r2 = Ref2(22)
+  r2.x.set(33) // ok
+  val r3: Ref2[Int] = Ref2(22)
+  r3.x.set(33) // error
+
+