1. Move the test generation to `CycleAnalyticsHelpers::TestGeneration`
2. Move all helper methods (previously placed in each individual spec
file) to `CycleAnalyticsHelpers`
Add a `before_end_fn` option to the code that generates cycle analytics
specs. `before_end_fn` is called before the end conditions are. Used for
data setup that needs to be called after the start conditions and before
the end conditions.
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`.
1. Generalise the specs that will be common across all cycle analytics
phases.
2. Rewrite specs `issue` and `plan` to use this abstracted testing
strategy.
3. Specs that are specific to a given phase, or unwieldy to test in an
abstracted manner, are added to each phase's spec.
