The status of a A/B testing request always eventually becomes "Failed"

The status of a A/B testing request always eventually becomes "Failed"
https://bugs.webkit.org/show_bug.cgi?id=141523

Reviewed by Andreas Kling.

The bug was caused by /api/build-requests always setting the status of a build request to 'failed' when
'failedIfNotCompleted' was sent by the buildbot sync'er.

Fixed the bug by only setting the status to 'failed' if it wasn't set to 'completed'.

* public/api/build-requests.php:
(main):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@180091 268f45cc-cd09-0410-ab3c-d52691b4dbfc