Eth2-to-NEAR-relay: use test configs for integration tests

[sept-en]

Currently, Eth2-to-NEAR-relay has integration tests that connect to Kiln endpoint and run tests for this network. We also have some pre-fetched files for some tests, network-specific. It makes sense to add a generalized case to utilize any network that is provided in the test config. Having that, we can later add in our CI multiple networks to the matrix and run tests for every available testing network and main network.