Migrate boxWithJava tests to multi-file framework

compiler/testData/codegen/boxWithJava/interfaces/inheritJavaInterface.kt

@@ -0,0 +1,20 @@
+// FILE: MyInt.java
+
+public interface MyInt {
+
+    String test();
+}
+
+// FILE: test.kt
+
+interface A : MyInt {
+    override public fun test(): String? {
+        return "OK"
+    }
+}
+
+class B: A
+
+fun box() : String {
+    return B().test()!!
+}