Fix flaky MergeRequestUserEntity test

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR fixes the use of let_it_be for :user since that object gets modified in line 49. Depending on the order that tests are executed, the test at spec/serializers/merge_request_user_entity_spec.rb:27 fails.

To test both scenarios, use bin/rspec 'spec/serializers/merge_request_user_entity_spec.rb' --seed 4890

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