Fix LDAP user leak into other tests

What does this MR do?

Fixes an issue where a test was changing Runtime::Env.ldap_username causing subsequent tests to fail

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/quality/nightly/issues/41

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
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