Fix: keep existing formatted value

[haase-fabian]

This PR allows for overriding the formattedValue via Field::setFormattedValue.

In my application I used something like this:

        yield CollectionField::new('foo')
            ->setFormattedValue("bar");

However it wasn't respected and I still got only the count of items in the datatable on index page.

~~This PR also sets the formattedValue, when the fields value was set manually. I don't think it is desired to not set the formattedValue at least equal to the value itself, even though the value was manually provided.~~

I also added a test case äquivilent to those for the Fields. It feels a bit weird to use AbstractFieldTest for a Configurator, however the necessary functionality was provided through set class.