Migrate boxMultiFile and boxMultifileClass tests to new multi-file tests

AbstractCompileKotlinAgainstMultifileKotlinTest is broken in this commit; will
be fixed later

compiler/testData/codegen/boxMultiFile/kt5445_2.kt

@@ -0,0 +1,23 @@
+// FILE: 1.kt
+
+package test2
+
+import test.A
+
+class C : A() {
+    fun a(): String {
+        return this.s
+    }
+}
+
+public fun box(): String {
+    return C().a()
+}
+
+// FILE: 2.kt
+
+package test
+
+open class A {
+    @JvmField protected val s = "OK";
+}