The zcml_layer can understand a "zcml_features" attribute, this attribute is set on the Layer that this test is in:

    >>> from van.testing.tests import ZCMLExtraLayer
    >>> ZCMLExtraLayer.zcml_features
    ('extra',)

Ths means our zcml was loaded with the "extra" extra and we can get the test_extra utility:

    >>> from zope.interface import Interface
    >>> from zope.component import queryUtility
    >>> queryUtility(Interface, name="test", default='None')
    'MARKER'
    >>> queryUtility(Interface, name="test_extra", default='None')
    'MARKER'
