Do not fail when subscription already exists (#3776)
Do not fail when subscription already exists (#3776)

Recently, we added a workaround for EOF errors:
we try to create a subscription multiple times when
it already exists.
Some test runs show errors like this:
```
creation.go:175: Creating v1 subscription
creation.go:180: Failed to create subscription "<name>": conversion webhook for messaging.knative.dev/v1beta1, Kind=Subscription failed: Post https://eventing-webhook.knative-eventing-aw90d392a6.svc:443/resource-conversion?timeout=30s: EOF
creation.go:175: Creating v1 subscription <name> for channel &TypeMeta{Kind:InMemoryChannel,APIVersion:messaging.knative.dev/v1,}-e2e-singleevent-channel-structured
creation.go:180: Failed to create subscription "<name>": subscriptions.messaging.knative.dev "e2e-singleevent-subscription-structured" already exists
creation.go:185: Failed to create subscription "<name>": subscriptions.messaging.knative.dev "e2e-singleevent-subscription-structured" already exists
```
So, this PR adds a check for already exists errors to
avoid failing tests in those cases.

Signed-off-by: Pierangelo Di Pilato <b6054fff5975a97a6b1347c22e6e7915c701af4c@gmail.com>
knative/eventing,knative/eventing