Remove generated codegen tests, move all testData to box/

A single test file will be generated out of box/ directory

compiler/testData/codegen/box/extensionProperties/inClassWithEmptySetter.kt

@@ -0,0 +1,13 @@
+class Test {
+  var Int.foo: String = "fail"
+
+  fun test(): String {
+    val i = 1
+    i.foo = "OK"
+    return i.foo
+  }
+}
+
+fun box(): String {
+   return Test().test()
+}