Add a test case, proving i15661 is fixed

tests/warn/i15661.scala

@@ -0,0 +1,7 @@
+case class Composite[T](l: List[T], v: T)
+
+def m(composite: Composite[?]): Unit =
+  composite match {
+    case Composite(l: List[Int], v: Int) => println(v) // warn: type test for List[Int] cannot be checked at runtime
+    case _ => println("This is not Int") // was: warn: Unreachable case except for null
+  }