Fix warnings in JVM codegen tests

Add generics where needed, add nullability annotations, etc.
Also delete some testcases from ArrayGenTest which will never be supported

compiler/testData/codegen/regressions/kt326.kt

@@ -1,51 +0,0 @@
-class List<T>(len: Int) {
-  val a = Array<T?>(len) { null }
-
-  fun reverse() {
-    var i = 0
-    var j = a.size-1
-    while(i < j) {
-        val x  = a[i]
-        a[i++] = a[j]
-        a[j--] = x
-    }
-  }
-}
-
-fun box() : String {
-  val d = List<Int>(1)
-  d.a[0] = 10
-  checkEquals(d.a[0], 10)
-
-  val a = List<String>(1)
-  a.a[0] = "1"
-  checkEquals(a.a[0], "1")
-
-  val b = List<Int?>(1)
-  b.a[0] = 10
-  checkEquals(b.a[0], 10)
-
-  val c = List<Array<Int>>(1)
-  c.a[0] = Array<Int>(4,{-1})
-  checkEquals(c.a[0]?.size, 4)
-
-  val e = List<Int>(5)
-  e.a[0] = 0
-  e.a[1] = 1
-  e.a[2] = 2
-  e.a[3] = 3
-  e.a[4] = 4
-  e.reverse()
-  for (i in 0..4)
-    checkEquals(e.a[i], 4-i)
-
-  return "OK"
-}
-
-fun checkEquals(a: Any?, b: Any?) {
-  if (a != b) throw AssertionError("Expected: $b, actual: $a")
-}
-
-fun main(args: Array<String>) {
-  println(box())
-}