Add test for trust proxy with function

[trivikr]

Confirmed that following branch is covered in coverage results:

fastify/fastify.js

Lines 220 to 222 in 6bce249

	if (typeof tp === 'function') {
	return tp
	}

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• commit message and code follows Code of conduct

[mcollina]

LGTM

[github-actions]

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