Set missing stages on ci builds to 'test' to avoid regressions in the pipelines view

## What does this MR do?

Sets a default stage on old `ci_builds`.

## Are there points in the code the reviewer needs to double check?

Is the migration acceptable for online execution?

## Why was this MR needed?

Old `ci_builds`with no stages cause a regression on the pipelines index view.

## What are the relevant issue numbers?

#17949 

Blocked by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4680


See merge request !4381