test: remove api mocks from link ms user spec

[ojeytonwilliams]

• test: seed the ms username to avoid mocking

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Rather than mocking the server's response, this modifies the db so that the api responds correctly.

This makes the e2e tests actually e2e tests as well as making them more useful for the api migration. i.e. if the migration breaks one of the routes and that route was mocked we wouldn't know.

[naomi-lgbt]

If we have to use a regex here, we should probably escape the wildcards just to be safe.

[ojeytonwilliams]

This will fail until #54518 is merged, so I've blocked it for now.