testserver: Fix pipelines update to set default for storage

[denik]

Why

This is the default that is set by backend if field is missing in Create/Update request. We already have Create covered, this applies it to Update.

Discovered while working on terraform removal #2926
Terraform reads the pipeline from the backend and then adds those defaults to future update requests while the new deployment backend does not read anything from the backend and those fields remain empty in update requests. The end result (as measured by "pipelines get $id") is, however, the same.