Merge boxMultiFile testData into box, delete BoxMultiFile test

compiler/testData/codegen/box/regressions/kt1515.kt

@@ -0,0 +1,30 @@
+// WITH_RUNTIME
+// FILE: 1.kt
+
+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 = Runnable::class.java
+    return (c.getName()!! == "java.lang.Runnable")
+}
+
+// FILE: 2.kt
+
+package otherpackage
+
+fun fromOtherPackage(): Boolean {
+    val c = Runnable::class.java
+    return (c.getName()!! == "java.lang.Runnable")
+}