Migrate boxWithJava tests to multi-file framework

compiler/testData/codegen/java8/box/defaultMethodOverride.kt

@@ -0,0 +1,23 @@
+// FILE: Simple.java
+
+public interface Simple {
+    default String test(String s) {
+        return s + "Fail";
+    }
+}
+
+// FILE: main.kt
+
+interface KInterface: Simple {
+    override fun test(s: String): String {
+        return s + "K"
+    }
+}
+
+class Test : KInterface {
+
+}
+
+fun box(): String {
+    return Test().test("O")
+}