Add regression spec for security bot create service

What does this MR do and why?

From Gitlab security bot creator fails to add bot to... (#426428 - closed):

In gitlab-com/gl-infra/production#16439 (comment 1579651800) we found that we were failing to assign created security bots to their projects.

We have created this MR to solve this problem and to avoid silently failing when bots are added to the project.

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