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/kt2331.kt

@@ -0,0 +1,14 @@
+class P {
+    var x : Int = 0
+        private set
+
+    fun foo() {
+        ({ x = 4 })()
+    }
+}
+
+fun box() : String {
+    val p = P()
+    p.foo()
+    return if (p.x == 4) "OK" else "fail"
+}