EZP-21783: Mark test skipped when class eZSolrMultiCoreBase is not available

In the `ezfSolrDocumentFieldBaseTest` an error is thrown when the class `eZSolrMultiCoreBase` is not present.

I don't know if it is not included in the Community Edition of eZFind or obsolete, but if it isn't present, the test should be skipped.

https://jira.ez.no/browse/EZP-21783

Cheers
:octocat: Jérôme