chore: add stress test

[karlkfi]

• Stress test tests 1,000 Namespaces, CofnigMaps, & CronTabs (CR)
• Stress test is a new test suite with its own make entrypoint
• Refactor shared test code so the e2e and stress tests can both use it
• Update test client QPS to 20 (from 5)

Use make test-stress to run the stress test. Takes ~15m on my machine (Apply + Destroy).

[Liujingfang1]

There is a merge conflict.

[karlkfi]

Weird... those pass locally.

E0311 01:23:04.735548   10292 task.go:263] Unknown object UID from ResourceCache (status: InProgress): default_foo_apps_Deployment
E0311 01:23:15.076888   10292 task.go:263] Unknown object UID from ResourceCache (status: InProgress): default_foo_apps_Deployment
E0311 01:23:15.077201   10292 task.go:263] Unknown object UID from ResourceCache (status: InProgress): default_secret__Secret
--- FAIL: TestApplier (11.13s)
    --- FAIL: TestApplier/apply_resource_with_existing_object_belonging_to_different_inventory (0.08s)
        applier_test.go:1499: Expected status event not received: &testutil.ExpStatusEvent{Identifier:object.ObjMetadata{Namespace:"default", Name:"foo", GroupKind:schema.GroupKind{Group:"apps", Kind:"Deployment"}}, Status:"Current", Error:error(nil)}
FAIL
coverage: 72.3% of statements
FAIL    sigs.k8s.io/cli-utils/pkg/apply 15.9[97](https://prow.k8s.io/view/gs/kubernetes-jenkins/pr-logs/pull/kubernetes-sigs_cli-utils/565/cli-utils-presubmit-master/1502092437772832768#1:build-log.txt%3A97)s

/test cli-utils-presubmit-master

[Liujingfang1]

Thank you for adding more tests!

/lgtm /approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: karlkfi, Liujingfang1

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