Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/properties/kt1892.kt

@@ -0,0 +1,5 @@
+val Int.ext : () -> Int = { 5 }
+val Long.ext : Long = 4.ext().toLong()  //(c.kt:4)
+val y : Long = 10.toLong().ext
+
+fun box() : String = if (y == 5.toLong()) "OK" else "fail"