Add safe-init test to prevent adding cold elements to hot arrays

This adds a negative test to ensure that the initialization checker does
nt allow cold values to be inserted into hot arrays through a method
call.

tests/init/neg/cold-insert-hot-array.scala

@@ -0,0 +1,9 @@
+object A:
+  def foo[T](x: T, array: Array[T]): Unit = array(0) = x
+
+class B {
+  var a = new Array[B](2)
+  A.foo(this, a)
+  println(a(0).i)
+  val i = 99
+}