When the thread is aborted, an exception is raised before the `start` of a job is set, and therefore raises an exception in the `ensure` block. This commit checks that `start` exists, and also adds `abort_on_exception=true` so that this issue would have caused test failures.
This will give us some aggregate stats on the defer queue performance.
It is limited to 100 entries (for safety) which is stored in an LRU cache.
Scheduler::Defer.stats can then be used to get an array that denotes:
- number of runs and completions (queued, finished)
- error count (errors)
- total duration (duration)
We can look later at exposing these metrics to gain visibility on the reason
the defer queue is clogged.
It's very easy to forget to add `require 'rails_helper'` at the top of every core/plugin spec file, and omissions can cause some very confusing/sporadic errors.
By setting this flag in `.rspec`, we can remove the need for `require 'rails_helper'` entirely.
