Add e2e test workflow

[tsaarni]

This PR introduces an end-to-end test that performs the following sequence:

1. Starts a Kind cluster
2. Installs the Contour Helm chart
3. Deploys the ingress-controller-conformance echoserver
4. Verifies that requests are successfully served through Contour
5. Uninstalls the Helm deployment

The test suite is based on Contour's e2e test suite and scripts, using Go and Ginkgo test framework. While it currently contains only a single minimal test case, it must be admitted this approach is not minimal. At this stage, a shell script could have been sufficient, but re-using Contour's test framework could be useful if more tests are expected in future. Some parts are not directly copied from Contour. For example, instead of using sigs.k8s.io/controller-runtime/pkg/client, it executes kubectl CLI (currently used for namespace creation and to deploy echoserver), since I assume there is no need for detailed programmatic access to Kubernetes resources.

Additional changes:

• Adds a Makefile to start the test.
• Introduces make target for checks, currently limited to Go linting (xref Feature: Add coding style and linting for the project #14)
• Adds workflow to install tools and execute all above for each pull request

I've included IPv6 related e2e scripts from Contour but IPv6 is not currently used in the test suite.

TODO

• Upgrade test between two chart versions

Fixes #13

[tsaarni]

Hi @projectcontour/contour-helm-chart-maintainers, and others, when you have a chance, could you please review this? Much appreciated!