Add E2E framework and initial set of simple tests, to run as part of PRs

[jgwest]

Argocd-agent currently has a set of detailed unit tests across the code base, which work well for white-box testing of individual functions within the packages of the project. We also have a what looks like a simple E2E test of principal code (but from quickly skimming the code it IMHO looks like a slightly more complex unit test?).

What is missing is fully E2E tests (and a framework to support them) that: setup vclusters, run Argo CD, run principal agent, run one or more of managed/autonomous agents, then test the resulting setup.

Criteria:

Initial brainstorm of how the test would be setup and what the test would do:

• Installs and starts k3s (same as we have done in e.g. Argo CD or Argo CD operator )
• Setup environment, similar to/based on hack/demo-env:
  • Creates vclusters vcluster-control-plane/vcluster-agent-managed/vcluster-agent-autonomous on that k3s instance
    • K3s appears to include a built-in LoadBalancer already, ServiceLB
  • Installs Argo CD to vclusters
  • Starts principal
  • Starts autonomous/managed agents (do we want to start both, or should we only start one or the other)?
  • Logic for all of this can be borrowed from https://github.com/jannfis/argocd-agent/tree/main/hack/demo-env
• Runs E2E tests
  • A simple managed-mode E2E test
  • A simple autonomous-mode E2E test
  • I'd imagine these simple E2E test would be something like:
    • Create an Argo CD Application in principal, then verify it is synced/deployed on managed-mode/autonomous agent vcluster
    • Verify the resources exist on those target clusters
  • Cleans up when done
• E2E tests to be written with testify, similar to the other unit tests already in the codebase.
• For E2E tests I'm always a fan of using a fixture (fixture implementation), which is a set of packages that contain utility functions that can be reused for writing new E2E tests.
  • Or for something even fancier, you can see what argo cd does for their E2E tests
• GitHub action to run the test for GitHub PRs (can steal this from .github folder of argocd-operator, argo-cd, etc)