Merge boxMultiFile testData into box, delete BoxMultiFile test

compiler/testData/codegen/box/sam/constructors/runnableAccessingClosure2.kt

@@ -0,0 +1,11 @@
+class Box(val s: String) {
+    fun extract(): String {
+        var result = ""
+        Runnable { result = s }.run() // capturing this and local var
+        return result
+    }
+}
+
+fun box(): String {
+    return Box("OK").extract()
+}