Move blackBoxFile() testData to box/ directory

Delete all test methods (and empty test classes), since they'll be
auto-generated

compiler/testData/codegen/box/classes/kt1891.kt

@@ -0,0 +1,14 @@
+class MyList<T>() {
+    var value: T? = null
+
+    fun get(index: Int): T = value!!
+
+    fun set(index: Int, value: T) { this.value = value }
+}
+
+fun box(): String {
+    val list = MyList<Int>()
+    list[17] = 1
+    list[17] = list[18]++
+    return "OK"
+}