Add GitHub workflows for testing

[mogren]

What type of PR is this?

feature

Which issue(s) this PR fixes:

We want to enable e2e tests on PRs before they are merged. (This PR replaces #1194 with a simpler solution)

What this PR does:

• Add a standard unit test run for all PRs
• Add integration tests for PRs that run on on a manual workflow trigger
• Add a nightly test run.

To do this we use the recently added pull_request_target that always runs in the context of the base repository for the PR.

The manual workflow is triggered by a workflow_dispatch event that is only available to users with write permissions to the repo.

This PR also does the first step of #786, disabling the NLB and ALB tests on PR e2e tests, but run them in the nightly tests by setting RUN_TESTER_LB_ADDONS=true.

Testing:

Manual e2 run triggered:
https://github.com/mogren/amazon-vpc-cni-k8s/runs/1074412555?check_suite_focus=true

Tests on in-repo branch PRs works:
https://github.com/mogren/amazon-vpc-cni-k8s/runs/1069555238?check_suite_focus=true

Nightly test run:
https://github.com/mogren/amazon-vpc-cni-k8s/runs/1072813881?check_suite_focus=true

The changes to the run-integration-tests.sh script still works with CircleCI:
https://app.circleci.com/pipelines/github/mogren/amazon-vpc-cni-k8s/883/workflows/33505350-bd9a-437b-99e0-526e678ae4e8/jobs/1787

Automation added to e2e:
This PR runs the e2e tests as GitHub workflows

Will this break upgrades and downgrades. Has it been tested?:
It will not

Does this require config changes:
No CNI config changes, but it does require adding self hosted runners to the aws/amazon-vpc-cni-k8s repo.

Does this PR introduce a user-facing change?:
No

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[fawadkhaliq]

Looks good overall. Let's add clean up as part of workflow here so anything (e.g Docker images) created as part of the test are cleaned up (from the runner) at the end of the test. Alternatively, you could have this cleaned up before every test run

[mogren]

Looks good overall. Let's add clean up as part of workflow here so anything (e.g Docker images) created as part of the test are cleaned up (from the runner) at the end of the test. Alternatively, you could have this cleaned up before every test run

Sounds good to clean them up before running the e2e tests. Added a docker image prune -f run:

 Clean up stale docker images
  docker image prune -f
Total reclaimed space: 0B

[fawadkhaliq]

/lgtm