Make query counting test more robust

This spec has proven fragile https://gitlab.com/gitlab-org/gitlab-ce/blob/master/spec/serializers/pipeline_serializer_spec.rb#L114. The query count has been tweaked up and down, depending on CE/EE and other unrelated factors.

Can we use https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/query_recorder.md to make this test more robust? If not, can we use a less-than expectation instead of a within 1?

There was some discussion on the original MR https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10462#note_26997653, and on Slack https://gitlab.slack.com/archives/C0XM5UU6B/p1502488096781708.