Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/arrays/collectionAssignGetMultiIndex.kt

@@ -0,0 +1,14 @@
+import java.util.ArrayList
+
+fun ArrayList<String>.get(index1: Int, index2: Int) = this[index1 + index2]
+fun ArrayList<String>.set(index1: Int, index2: Int, elem: String) {
+    this[index1 + index2] = elem
+}
+
+fun box(): String {
+    val s = ArrayList<String>(1)
+    s.add("")
+    s[1, -1] = "O"
+    s[2, -2] += "K"
+    return s[2, -2]
+}