Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/functions/kt785.kt

@@ -0,0 +1,13 @@
+class A() {
+    var x : Int = 0
+
+    var z = { () ->
+        x++
+    }
+}
+
+fun box() : String {
+    val a = A()
+    a.z()  //problem is here
+    return if (a.x == 1) "OK" else "fail"
+}