Add a test of 'product info' data returned from the management API

WildFly Core's org.wildfly.core.test.standalone.mgmt.api.core.ProductInfoUnitTestCase tests some of the data associated with the 'product info' in the root management resource, but not the items set by the org.jboss.as.product module. Add a test that the expected data is returned.

I should have done this as part of WFLY-18578, but my testing there was that a fake 'override' took effect, with no testing that the usual value is correct.