testserver: Fix pipelines update to set default for storage (#2960)

## 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.

Author: denik

libs/testserver/pipelines.go

@@ -40,20 +40,14 @@ func (s *FakeWorkspace) PipelineCreate(req Request) Response {
 	r.Spec = &spec
 
 	pipelineId := uuid.New().String()
-	spec.Id = pipelineId
 	r.PipelineId = pipelineId
 	r.CreatorUserName = "tester@databricks.com"
 	r.LastModified = time.Now().UnixMilli()
 	r.Name = r.Spec.Name
 	r.RunAsUserName = "tester@databricks.com"
 	r.State = "IDLE"
 
-	// If the pipeline definition does not specify a catalog, it switches to Hive metastore mode
-	// and if the storage location is not specified, API automatically generates a storage location
-	// (ref: https://docs.databricks.com/gcp/en/dlt/hive-metastore#specify-a-storage-location)
-	if spec.Storage == "" && spec.Catalog == "" {
-		spec.Storage = "dbfs:/pipelines/" + pipelineId
-	}
+	setSpecDefaults(&spec, pipelineId)
 	s.Pipelines[pipelineId] = r
 
 	return Response{
@@ -63,11 +57,21 @@ func (s *FakeWorkspace) PipelineCreate(req Request) Response {
 	}
 }
 
+func setSpecDefaults(spec *pipelines.PipelineSpec, pipelineId string) {
+	spec.Id = pipelineId
+	// If the pipeline definition does not specify a catalog, it switches to Hive metastore mode
+	// and if the storage location is not specified, API automatically generates a storage location
+	// (ref: https://docs.databricks.com/gcp/en/dlt/hive-metastore#specify-a-storage-location)
+	if spec.Storage == "" && spec.Catalog == "" {
+		spec.Storage = "dbfs:/pipelines/" + pipelineId
+	}
+}
+
 func (s *FakeWorkspace) PipelineUpdate(req Request, pipelineId string) Response {
 	defer s.LockUnlock()()
 
-	var request pipelines.PipelineSpec
-	err := json.Unmarshal(req.Body, &request)
+	var spec pipelines.PipelineSpec
+	err := json.Unmarshal(req.Body, &spec)
 	if err != nil {
 		return Response{
 			Body:       fmt.Sprintf("internal error: %s", err),
@@ -82,7 +86,8 @@ func (s *FakeWorkspace) PipelineUpdate(req Request, pipelineId string) Response
 		}
 	}
 
-	item.Spec = &request
+	item.Spec = &spec
+	setSpecDefaults(&spec, pipelineId)
 	s.Pipelines[pipelineId] = item
 
 	return Response{