Add test for gitlab:truncate_logs task

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What does this merge request do and why?

Ref: #2141 (closed)

This change add a rspec test file to test:

• rake gitlab:truncate_logs

How to set up and validate locally

From gdk root directory, enter bundle exec rspec spec/tasks/gitlab_spec.rb

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.