Use independent user for impersonating

What does this MR do?

This MR makes the "Manage Instance Start and stop user impersonation" independent by not using gitlab-qa-user1 and instead creating the user at runtime in an attempt to solve the 404 seen when the admin tries to impersonate the user.

Related to: #255894 (closed)

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team