Fix the synthetic-companion scripted test

As documented in `./test`, compilation should fail because `object A` is no
longer a subclass of `Int => A`, but `B.scala` did not actually check
that previously, so compilation always succeeded.

Author: smarter

sbt-dotty/sbt-test/source-dependencies/synthetic-companion/B.scala

@@ -1,3 +1,4 @@
 object B {
-  A(0)
+  val f: Int => A = A
+  f(0)
 }