fix(metadata): allow to use constants in XML configuration (resource attribute)

[alanpoulain]

Q	A
Branch?	2.6
Tickets	N/A
License	MIT
Doc PR	N/A

Allow to use constants in XML configuration, like this:

<?xml version="1.0" encoding="UTF-8" ?>
<!-- api/config/api_platform/resources.xml -->

<resources
        xmlns="https://api-platform.com/schema/metadata"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="https://api-platform.com/schema/metadata
        https://api-platform.com/schema/metadata/metadata-2.0.xsd">
    <resource class="App\Entity\Book">
        <attribute name="url_generation_strategy" type="constant">ApiPlatform\Core\Api\UrlGeneratorInterface::ABS_URL</attribute>
    </resource>
</resources>

Enable again the tests for the XML / YAML configuration, disabled since a long time.