Fix test and add check file

Author: odersky

tests/run/boehm-berarducci.check

@@ -0,0 +1,8 @@
+List(1, 2)
+List(2)
+1
+false
+List(1, 2)
+List(2)
+1
+false

tests/run/boehm-berarducci.scala

@@ -68,7 +68,7 @@ object ListOps2 extends ListOps:
 
   def fst[T](v: ListView[T]): T = v(hd => tl => hd)(() => ???)
   def snd[T](v: ListView[T]): List[T] = v(hd => tl => tl)(() => ???)
-  def isPair[T](v: ListView[T]): Boolean = v(hd => tl => false)(() => true)
+  def isPair[T](v: ListView[T]): Boolean = v(hd => tl => true)(() => false)
 
 @main def Test() =
   ListOps1.test()