Fix typo in `test_bp_signup_enabled_when_registration_setting_does_not_exist_should_default_to_true()`.

It was causing a reference error.

See [9219], [9220].

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9223 cdf35c40-ae34-48e0-9cc9-0c9da1808c22