Some of these back-ends require the url option as listed below.
+webtest_varname (**optionall**, string)
+ The name of the attribute to use when stashing the session object into
+ the environ for use with WebTest. The name provided here is where the
+ session object will be attached to the WebTest TestApp return value.
url (**optional**, string)
URL is specific to use of either ext:memcached or ext:database. When using
one of those types, this option is **required**.