Integration tests setup

[crypto-facs]

Description

Started migrating some parts from the setup @yihuang had done in cronos to better test RPC endpoints with multiple configurations.

Tests are written in python to take advantage of pystarport for node configuration and also because of the extensive w3 library support for easy testing. Nix was used to manage reproducible builds and easy composability of dependencies.

This configurations currently allows to setup a basic ethermint and geth node and expect tests to pass on both networks. At the moment I just added a super simple test_basic to prove the concept as the configuration itself is very complex.

Openning as a first draft to get some comments on it and as a foundation on which to build a robust test suite to maintain better reliabily on the RPC nodes. I will add extensive tests once this PR is merged and also other several configurations on which to tests like pruned_nodes, which should be straightforward with this suite as it has been done in Cronos.

Big shoutout to @yihuang for setting this up

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[yihuang]

LGTM, left small cleanup suggestions.

[fedekunze]

@crypto-facs can you move the tests to the existing tests/ directory?

[yihuang]

LGTM