WL-3170 Test can define `vevent_sequence`

We should be able to define the sequence of an event through the
"vevent_sequence" parameter.

It is used this way in signup.
WL-3170 Cancelled events have sequence

The sequence property needs to be defined on events within cancelled
calendars. We should define the sequence when constructing the event but
if we come to the point of cancelling an event which doesn't have sequence
defined then lets add it in for good measure.