Migrate boxMultiFile and boxMultifileClass tests to new multi-file tests

AbstractCompileKotlinAgainstMultifileKotlinTest is broken in this commit; will
be fixed later

compiler/testData/codegen/boxMultiFile/kt1515.kt

@@ -0,0 +1,29 @@
+// 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")
+}