While it could be argued we're hiding the real way we've defined
associations in our models, the tests are so much easier to read when we
don't have so many lines just creating data.
Furthermore, developers who care about vertically aligning the code will
be glad to see some variables disrupting this alignment are now gone.
Now factories define default headings for investments, so there's no
need to create a group and a heading to create an investment.
Likewise, in order to create a heading it isn't necessary to specify a
group anymore; specifying the budget is enough.
It's possible that there are more similar cases we haven't simplified
yet; I'm only addressing the obvious ones.
The `type: :feature` is automatically detected by RSpec because these
tests are inside the `spec/features` folder. Using `feature` re-adds a
`type: :feature` to these files, which will result in a conflict when we
upgrade to Rails 5.1's system tests.
Because of this change, we also need to change `background` to `before`
or else these tests will fail.
