Migrate boxMultiFile and boxMultifileClass tests to new multi-file tests

AbstractCompileKotlinAgainstMultifileKotlinTest is broken in this commit; will
be fixed later

compiler/testData/codegen/boxMultiFile/accessorForProtected.kt

@@ -0,0 +1,38 @@
+// FILE: 1.kt
+
+import b.B
+import a.BSamePackage
+
+fun box() = if (B().test() == BSamePackage().test()) "OK" else "fail"
+
+// FILE: 2.kt
+
+package a
+
+open class A {
+    protected fun protectedFun(): String = "OK"
+}
+
+class BSamePackage: A() {
+    fun test(): String {
+        val a = {
+            protectedFun()
+        }
+        return a()
+    }
+}
+
+// FILE: 3.kt
+
+package b
+
+import a.A
+
+class B: A() {
+    fun test(): String {
+        val a = {
+            protectedFun()
+        }
+        return a()
+    }
+}