These tests were failing randomly because there is no guarantee
the methods `Budget#email_selected` and `Budget#email_unselected` will
always send the emails in the same order, because `investments.selected`
and `investments.unselected` don't necessarily return the records in the
order they were created.
Ordering by id is certainly not very elegant; however, ordering by
another field like `created_at` is dangerous because the record could be
created at (almost) the exact same time.
Related to issue #2446 and issue #2519.
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