Fix flaky tests in work items feature specs

What does this MR do and why?

Related to gitlab-org/quality/engineering-productivity/master-broken-incidents#5971 (closed)

Fix flaky tests in spec/features/projects/work_items/work_item_spec.rb

Instead of assigning the user sole within a before block, sign in with a user that already have that role. Also in the assignees examples assign a different user than the current user, unless we are testing the assign yourself option.

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