Create and use a new user to avoid role based conflict

What does this MR do?

These tests were adding gitlab_qa_username_1 as a maintainer to the sandbox group. This meant that in a shared/live environment, any other test could not add gitlab_qa_username_1 in a role lower than maintainer (such as Guest) to a subgroup or project in the sandbox group.

This MR changes that behaviour by creating and using new users in those tests.

Closes: #232599 (closed) #232722 (closed) #232886 (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