Migrate boxAgainstJava tests to multi-file framework

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

@@ -0,0 +1,26 @@
+// FILE: JavaClass.java
+
+public class JavaClass {
+    private String x = null;
+
+    public String getX() { return "OK"; }
+    protected void setX(String x) { this.x = x; }
+}
+
+// FILE: 1.kt
+
+package p
+
+import JavaClass
+
+fun box(): String {
+    return KotlinClass().ok()
+}
+
+class KotlinClass : JavaClass() {
+    fun ok(): String {
+        x = "o"
+        x += "k"
+        return x
+    }
+}