Update fixtures to remove hardcoded organization names

[erinysong]

Issue description

fixtures_requests.py still hardcodes a string organization_name for our created Domain Requests. Now that organization name is tied to Portfolios, this leads to some some fixtures Domain Requests' organization names being incorrect.

Domain Requests created within a Portfolio still have the organization name as a hardcoded value from fixtures_requests.py (eg "Example - Withdrawn") instead of the actual portfolio's organization name. This generally gets corrected when assigning an organization name to a Domain Request on the UI, but in the edge case we assign a suborganization, instead of an organization, to a Domain Request the error persists.

Acceptance criteria

• Update fixtures so that Domain Requests' organizations reflect the portfolio they are assigned to in fixtures.

Additional context

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Links to other issues

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