Fix flow.json fork field usage in flow test --fork <CUSTOM_FORK_NETWORK>

[jribbink]

Closes #2244

Description

This new syntax is recognized for flow emulator --fork mainnet-fork:

"networks": {
    "mainnet": {
        "mainnet-fork": {
            "host": "127.0.0.1:3569",
            "fork": "mainnet"
        }
    }
}

This PR adds the missing piece preventing the following syntax from being recognized in the flow test command.

The test framework forking guides/usage are currently advising users to use the syntax flow test --fork mainnet which can potentially be too rigid for complex testing scenarios divergent from mainnet (the original idea behind introducing custom networks).

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[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

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[codecov-commenter]

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✅ All modified and coverable lines are covered by tests.

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