Move multi-file codegen testData to boxMultiFile/

There'll be an auto-generated test over this directory

compiler/testData/codegen/boxMultiFile/kt2060/1.kt

@@ -0,0 +1,12 @@
+import testing.ClassWithInternals
+
+public class HelloServer() : ClassWithInternals() {
+    public override fun start() {
+        val test = foo() + someGetter //+ some
+    }
+}
+
+fun box() : String {
+    HelloServer().start()
+    return "OK"
+}