Fix flaky tests in stats package

[sharanyad]

Summary

Fix flaky unit tests in stats package due to a call in init()
Fixes #1019

Implementation details

Remove unnecessary ecsengine.NewDockerGoClient call in init method of unit tests and add to integ test

Testing

• Builds on Linux (make release)
• Builds on Windows (go build -out amazon-ecs-agent.exe ./agent)
• Unit tests on Linux (make test) pass
• Unit tests on Windows (go test -timeout=25s ./agent/...) pass
• Integration tests on Linux (make run-integ-tests) pass
• Integration tests on Windows (.\scripts\run-integ-tests.ps1) pass
• Functional tests on Linux (make run-functional-tests) pass
• Functional tests on Windows (.\scripts\run-functional-tests.ps1) pass

New tests cover the changes:

Description for the changelog

Licensing

This contribution is under the terms of the Apache 2.0 License: yes