Integration tests

[kiseln]

These tests deploy light client contract to near sandbox and run relayer process. In the end we check that certain amount of blocks were added to the light client.

Looking for feedback on the following:

• Configuration values for different blockchains (batch_size especially)
• Which RPCs we want to use
• Any specific scenarios that we want to test. However, I think it's more suitable for integration tests to cover basic scenario and use unit testing for advanced logic