IMipPlugin: Added unit test for dav.invitation_link_recipients support

Existing tests required modification to correctly mock up the new config
parameter fetch, and to set the RSVP flag for attendees (since the test
was detecting token generation, and we no longer generate tokens when
no RSVP is requested by the client or sent by nextcloud).

Signed-off-by: Brad Rubenstein <brad@wbr.tech>