Introduce a flag to disable copying pull secret in e2e TS

[mvinkler]

Problem
At this point, it is not possible to run eventing e2e TS with a regular user on Openshift.
There is #5357 as a main issue, but there is also this side issue - a regular user cannot copy kn-eventing-test-pull-secret pull secret here: https://github.com/knative/eventing/blob/main/test/lib/creation.go#L526
The related comment says that this is needed for cases where the images are in a private registry, so it makes sense to be able to skip this step.

Persona:
Which persona is this feature for?

Exit Criteria
kn-eventing-test-pull-secret pull secret is not copied during the eventing e2e TS run.

[mgencur]

@mvinkler Do you think pre-creating the namespaces and copying the pull-secret in advance by admin user would resolve this? The tests could skip this step if the pull-secret is already there.

[mvinkler]

If a regular user can check if a pull-secret is already present, then it would work.

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[mgencur]

Just a note. At least for Conformance tests this has been resolved in #5596 . The point is that the admin user will create the namespaces in advance and also create/copy the pull secret. If the flag -reusenamespace is passed then copying the secret is skipped.

[github-actions]

This issue is stale because it has been open for 90 days with no
activity. It will automatically close after 30 more days of
inactivity. Reopen the issue with /reopen. Mark the issue as
fresh by adding the comment /remove-lifecycle stale.