Geth test integration

[mederic-p]

Issue/Task Number: 1089
Closes #1089

Overview

This PR adds a few integration tests using geth.
Geth must be installed on the machine to successfully run these tests.
Note that I haven't added/changed anything to circle ci pipeline so it won't run these integration tests.

Changes

• Add a few integration tests.

Implementation Details

A geth node will be started for each integration test using the --dev option.
Each test takes about 7 sec to run.

Usage

The existing tests can be ran as usual with mix test and the new integration tests can be ran using mix test --only integration

[unnawut]

Lovely <3 Some small comments.

[sirn]

For the CI pipeline, I think the easiest way to do it is to attach Geth as a sidecar in executors.builder_pg (probably rename it to builder_intg or something), e.g. something among the line of:

- image: ethereum/client-go:release-1.9
  entrypoint:
    - geth
    - --dev

Default Geth Dockerfile should expose port 8545, 8546 and 30303 by default, so you can access it via localhost:8545, etc. in the test step. E2E on the other hand rely on docker-compose which you can just add geth as another container in docker-compose.

[T-Dnzt]

Approving, but probably missing a warning when Geth is already running. If that's the case, the test will silently fail to start Geth and use the currently running one, resulting in unpredictable stuff happening.