[BUG] Fix failing unit test

[peytondmurray]

Fixes #991.

Description

This pull request:

• Fixes the return code expected by test_conda_store_server_url_prefix

Pull request checklist

• [x] Did you test this change locally?
• [x] Did you update the documentation (if required)?
• [x] Did you add/update relevant tests for this change (if required)?

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[peytondmurray]

Closing in favor of https://github.com/conda-incubator/conda-store/pull/993.