Migrate boxAgainstJava tests to multi-file framework

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

@@ -1,3 +1,19 @@
+// FILE: JavaClass.java
+
+class JavaClass {
+    private int myX = 0;
+
+    public int getX() {
+        return myX;
+    }
+
+    public void setX(int x) {
+        myX = x;
+    }
+}
+
+// FILE: 1.kt
+
 fun box(): String {
     val javaClass = JavaClass()
     javaClass.x++