Fix nil pipeline status error

What does this MR do and why?

Describe in detail what your merge request does and why.

Sometimes there is latency in pipeline creation that results in

Failure/Error: new_pipeline[:status] == 'success' && downstream_project.pipelines.size == 2

NoMethodError:
  undefined method `[]' for nil:NilClass

Updating test to wait until pipeline response is not nil first then check for status.

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Related to #328188 (closed)