Remove overlap from the "start + end" durations in the happy test
case. For the `staging` phase, the end time is the _first_ deployment
that happens after the MR merge.

If we have 5 MRs where the `start_time`s (merge time) are the
same, and all the `end_time`s (deploy to production) a few days from
now, only the earliest deploy will get picked up, because that
consitutes a deploy for _all_ the MRs.

We fix this by removing overlap. Every `start_time` is now generated to
be _after_ the preceding `end_time`.