Retry on webhook errors in rekt tests

[pierDipi]

Describe the bug

We have a workaround for knative/pkg#1509 in the
existing eventing testing library that is currently not used in rekt:

eventing/test/lib/creation.go

Lines 55 to 72 in a8a3063

	// This is a workaround for https://github.com/knative/pkg/issues/1509
	// Because tests currently fail immediately on any creation failure, this
	// is problematic. On the reconcilers it's not an issue because they recover,
	// but tests need this retry.
	//
	// https://github.com/knative/eventing/issues/3681
	func isWebhookError(err error) bool {
	return strings.Contains(err.Error(), "eventing-webhook.knative-eventing")
	}
	func (c *Client) RetryWebhookErrors(updater func(int) error) error {
	attempts := 0
	return retry.OnError(retry.DefaultRetry, isWebhookError, func() error {
	err := updater(attempts)
	attempts++
	return err
	})
	}

Expected behavior

Mitigation for knative/pkg#1509

To Reproduce

N/A

Knative release version

Additional context

/area test-and-release