test: Add unit tests + ci to test the txfetcher

[ferranbt]

📝 Summary

This PR adds a unit test to check the end-to-end functionality of the transaction fetcher. It uses an anvil node as an Eth client to subscribe to its transactions. Adding also anvil to the CI/CD pipeline.

The setup for the test is a bit verbose now but I imagine that if this pattern becomes useful (unit testing with a real Eth node) we will create more concrete util abstractions.

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✅ I have completed the following steps:

• [x] Run make lint
• [x] Run make test
• [x] Added tests (if applicable)

[github-actions[bot]]

Benchmark results for 5ffff43

Report: https://flashbots-rbuilder-ci-stats.s3.us-east-2.amazonaws.com/benchmark/5ffff43-f468712/report/index.html

Date (UTC)	2024-07-03T13:07:18+00:00
Commit	5ffff4307b6c0730439a514913503428ed3c2309
Base SHA	f46871217ce85a9f9082663140544bcfe7e6685f

Significant changes

Benchmark	Mean	Status
MEV-Boost SubmitBlock serialization/JSON encoding	4.06%	Performance has degraded.