Migrate boxAgainstJava tests to multi-file framework

compiler/testData/codegen/boxAgainstJava/syntheticExtensions/protected.kt

@@ -0,0 +1,19 @@
+// FILE: JavaClass.java
+
+public class JavaClass {
+    protected String getOk() { return "OK"; }
+}
+
+// FILE: 1.kt
+
+package p
+
+import JavaClass
+
+fun box(): String {
+    return KotlinClass().ok()
+}
+
+class KotlinClass : JavaClass() {
+    fun ok() = ok
+}