Minor, move tests on KT-39054 into a subdirectory

compiler/testData/codegen/box/delegatedProperty/delegateToAnother/mutable.kt

@@ -0,0 +1,13 @@
+// WITH_STDLIB
+// IGNORE_BACKEND: JS
+class C(var x: String)
+
+var x = "fail"
+var y by ::x
+var z by C("fail")::x
+
+fun box(): String {
+    y = "O"
+    z = "K"
+    return y + z
+}