Simplify end-to-end test setup

What does this MR do?

Simplifies an end-to-end test setup.

Besides that, this MR tries to fix an issue that happens on staging (see Slack thread), where user fabrication via API doesn't work, and it may be due to the usage of let(:user) since other tests using User.fabricate_via_api! don't use it in a let declaration.

In this commit, I'm moving the user fabrication to the before block and exposing it as an instance variable @user, so that it can be used in the test itself.

Also, I have removed unnecessary steps that make the test slower.

Finally, with this change, we have 14 fewer lines of code.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content