Migrate boxWithJava tests to multi-file framework

compiler/testData/codegen/boxWithJava/properties/protectedJavaProperty.kt

@@ -0,0 +1,36 @@
+// FILE: JavaBaseClass.java
+
+public class JavaBaseClass {
+
+    private String field = "fail";
+
+    protected String getFoo() {
+        return field;
+    }
+
+    protected void setFoo(String foo) {
+        field = foo;
+    }
+}
+
+// FILE: kotlin.kt
+
+package z
+
+import JavaBaseClass
+
+object KotlinExtender : JavaBaseClass() {
+    @JvmStatic fun test(): String {
+        return runSlowly {
+            foo = "OK"
+            foo
+        }
+    }
+}
+fun runSlowly(f: () -> String): String {
+    return f()
+}
+
+fun box(): String {
+    return KotlinExtender.test()
+}