Fix quarantined spec in count_imported_projects_metric_spec.rb by chronogically creating db records

What does this MR do and why?

Related to #362591 (closed).

This fixes the quarantined test spec/lib/gitlab/usage/metrics/instrumentations/count_imported_projects_metric_spec.rb[1:2:2:1:1:1] by ensuring that test records are created chronologically by created_at, because the metric SQL query optimization relies heavily on the fact that ids increment monotonically through time.

Explanation of the bug: #362591 (comment 982484850)

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