Migrate boxMultiFile and boxMultifileClass tests to new multi-file tests

AbstractCompileKotlinAgainstMultifileKotlinTest is broken in this commit; will
be fixed later

compiler/testData/codegen/boxMultiFile/kt2060.kt

@@ -0,0 +1,28 @@
+// FILE: 1.kt
+
+import testing.ClassWithInternals
+
+public class HelloServer() : ClassWithInternals() {
+    public override fun start() {
+        val test = foo() + someGetter //+ some
+    }
+}
+
+fun box() : String {
+    HelloServer().start()
+    return "OK"
+}
+
+// FILE: 2.kt
+
+package testing; // There is no error if both files are in default package
+
+public abstract class ClassWithInternals {
+   protected var some: Int = 0;
+   protected var someGetter: Int = 0
+      get() = 5
+
+   protected fun foo() : Int = 0
+
+   public abstract fun start() : Unit;
+}