Kbauer/setup nr backend tests

[kb-newrelic]

Summary

• Setup new e2e test which exports data to NR (instead of using a mock backend). For the lack of better terminology, I decided to call these new tests 'slow e2e' and the existing mock backend tests 'fast e2e'. Open to changing this (including the dir/file structure).
  • leverages newrelic-client-go for API calls
• Slow tests run on a dedicated nightly CI build (ci_nightly.yaml) using main and can be manually triggered with any branch/commit
  • local run of slow tests worked as expected (ingested data, verification call)
• Main work of this PR was generalizing the current test setup:
  • moved charts to separate dir (test/charts) to allow reuse of charts but also not tie charts to specific test
  • define second chart (test/charts/nr_backend) with NR as backend
  • move duplicate test logic into various types of util functions (test/e2e/util). Open to rehauling this, just a first attempt.
• Necessary secrets (ingest key, api key, staging urls,...) are passed around as env vars and then turned into k8s Secrets

Misc

• Test tagging (slow/fast) currently done via go test -short flag. Go community doesn't seem to have a clear consensus to follow this pattern or to use env vars instead. Wrapped it in test/e2e/util/test to make changing it easier down the line if we decide to do so.

[mailo-nr]

LGTM