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/innerClass.kt

@@ -0,0 +1,19 @@
+class Outer(val foo: StringBuilder) {
+  inner class Inner() {
+    fun len() : Int {
+      return foo.length()
+    }
+  }
+
+  fun test() : Inner {
+    return Inner()
+  }
+}
+
+fun box() : String {
+  val sb = StringBuilder("xyzzy")
+  val o = Outer(sb)
+  val i = o.test()
+  val l = i.len()
+  return if (l != 5) "fail" else "OK"
+}