Test transaction from extension with `appsignal_transaction_to_json`

[tombruijn]

• [x] Implement appsignal_transaction_to_json in the Gem's C-extension #291
• [ ] Add it to tests to check if all the assertions are correct.
• [ ] Add test mode to extension so that we don't have to stub the complete call

See: https://github.com/appsignal/appsignal-agent/pull/228

[thijsc]

appsignal_transaction_to_json can return an empty string, you should interpret that as nil. I'd return Qnil from the C glue code in that case.

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