Move ir box test under "box/ir"

compiler/testData/codegen/box/ir/closureConversion/mutable1.kt

@@ -0,0 +1,9 @@
+fun box(): String {
+    var result = ""
+    fun add(s: String) {
+        result += s
+    }
+    add("O")
+    add("K")
+    return result
+}