Migrate boxAgainstJava tests to multi-file framework

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

@@ -1,3 +1,18 @@
+// FILE: JavaClass2.java
+
+class JavaClass1 {
+    protected Object value = null;
+
+    public Object getSomething() { return null; }
+    public void setSomething(Object value) {  this.value = value; }
+}
+
+class JavaClass2 extends JavaClass1 {
+    public String getSomething() { return (String)value; }
+}
+
+// FILE: 1.kt
+
 fun box(): String {
     val javaClass = JavaClass2()
     javaClass.something = "OK"