Migrate boxMultiFile and boxMultifileClass tests to new multi-file tests

AbstractCompileKotlinAgainstMultifileKotlinTest is broken in this commit; will
be fixed later

compiler/testData/codegen/boxMultiFile/kt9717DifferentPackages.kt

@@ -0,0 +1,23 @@
+// FILE: a.kt
+
+package a
+
+import b.*
+
+fun box(): String {
+    BB().ok()
+    return BB().OK
+}
+
+// FILE: b.kt
+
+package b
+
+public open class B {
+    public var OK: String = "OK"
+        protected set
+}
+
+public class BB : B() {
+    public fun ok(): String = OK
+}