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/outerThis.kt

@@ -0,0 +1,12 @@
+class Outer() {
+  inner class Inner() {
+    val outer: Outer get() = this@Outer
+  }
+
+  public val x : Inner = Inner()
+}
+
+fun box() : String {
+  val o = Outer()
+  return if (o === o.x.outer) "OK" else "fail"
+}