Move multi-file codegen testData to boxMultiFile/

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

compiler/testData/codegen/boxMultiFile/kt1515/1.kt

@@ -0,0 +1,18 @@
+package thispackage
+
+import otherpackage.*
+
+fun box(): String {
+    if (!localUse()) {
+        return "local use failed"
+    }
+    if (!fromOtherPackage()) {
+        return "use from other package failed"
+    }
+    return "OK"
+}
+
+fun localUse(): Boolean {
+    val c = javaClass<Runnable>()
+    return (c.getName()!! == "java.lang.Runnable")
+}

compiler/testData/codegen/boxMultiFile/kt1515/2.kt

@@ -0,0 +1,6 @@
+package otherpackage
+
+fun fromOtherPackage(): Boolean {
+    val c = javaClass<Runnable>()
+    return (c.getName()!! == "java.lang.Runnable")
+}