Add SYSTEM_NAMESPACE introduction in test README

[zhangtbj]

Changes

Recently, there is a new change in the Knative test framework:
By default, the SYSTEM_NAMESPACE will be set as knative-testing:
https://github.com/knative/pkg/blob/67d950c438f17c46108761215c0c8394484edea5/system/testing/names.go#L29

So Tekton introduced this change, to set SYSTEM_NAMESPACE=tekton-pipelines during install_pipeline_crd:
https://github.com/tektoncd/pipeline/blob/master/test/e2e-common.sh#L30

But for our scenario, we deploy Tekton by ourselves and just need to run the E2E test by using go test directly:
https://github.com/tektoncd/pipeline/tree/master/test#end-to-end-tests

But if I don't run e2e-tests.sh I will miss this setting SYSTEM_NAMESPACE=tekton-pipelines which makes failure in some of E2E tests, like:
Failed to get ConfigMap feature-flags: configmaps "feature-flags" not found, because we don't have knative-testing namespace, so of course no configmaps "feature-flags" in there:
https://github.com/tektoncd/pipeline/blob/master/test/tektonbundles_test.go#L524

So I add this SYSTEM_NAMESPACE in the test README to help the person who wants to run the E2E by using go test directly.

/kind documentation

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Release Notes

[tekton-robot]

Hi @zhangtbj. Thanks for your PR.

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[vdemeester]

/ok-to-test

[dibyom]

/lgtm

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: sbwsg

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