Migrate boxWithJava tests to multi-file framework

compiler/testData/codegen/boxWithJava/interfaceDefaultImpls.kt

@@ -0,0 +1,20 @@
+// FILE: B.java
+
+class B {
+    static String test(A x) {
+        return A.DefaultImpls.foo(x);
+    }
+}
+
+// FILE: main.kt
+
+interface A {
+    fun foo() = "OK"
+}
+
+fun box(): String {
+    val result = B.test(object : A {})
+    if (result != "OK") return "fail: $result"
+
+    return "OK"
+}