Add capture-checking test for boundary

Author: natsukagami

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

@@ -0,0 +1,9 @@
+import language.experimental.captureChecking
+
+import scala.util.boundary
+
+object test:
+  boundary[boundary.Label[Unit]]: l1 ?=> // error // error // error // error
+    boundary[Unit]: l2 ?=>
+      boundary.break(l2)(using l1) // error
+    ???