Migrate boxMultiFile and boxMultifileClass tests to new multi-file tests

AbstractCompileKotlinAgainstMultifileKotlinTest is broken in this commit; will
be fixed later

compiler/testData/codegen/boxMultiFile/kt9717.kt

@@ -0,0 +1,12 @@
+// FILE: box.kt
+
+object Test {
+    val test: String = OK
+}
+
+fun box(): String = Test.test
+
+// FILE: Vars.kt
+
+public var OK: String = "OK"
+    private set