[DependencyInjection] Fixed missing 'factory-class' attribute in XmlDumper output

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Problem: XmlDumper doesn't write 'factory-class' XML attribute for definitions on which setFactoryClass() was called.

Impact: Container[Builder] to throws an exception when the relevant service is being requested/initiated after loading the dumped XML.
`Uncaught Exception Symfony\Component\DependencyInjection\Exception\RuntimeException: "Cannot create service "xxx" from factory method without a factory service or factory class." at /<path>/<to>//DependencyInjection/ContainerBuilder.php`

Solution: Made XmlDumper write the 'factory-class' attribute, and updated the relevant test fixture.

Another related problem, is that XMLFileLoader doesn't complain if the 'factory-class' attribute is missing for a 'service' elements that include 'factory-method' attribute, resulting in an ill-configured Definition object in the ContainerBuilder. I'll post an issue/ticket, and probably send another PR for that.