Fix fn test failed to query logs from log database (#1401)

When `fn test` failed to retrieve logs from the function
pod, it turns to query logs from the log database. However,
the logdb type flag is not set to `fn test`, hence the logdb
type is empty (even without default value) and cause `fn logs`
returns an error due to unable to find the corresponding log
database type.

This PR adds logdb type flag to `fn test` to resolve the problem.

pkg/fission-cli/cmd/function/log.go

@@ -73,7 +73,7 @@ func (opts *LogSubCommand) do(input cli.Input) error {
 	// request the controller to establish a proxy server to the database.
 	logDB, err := logdb.GetLogDB(dbType, server)
 	if err != nil {
-		return errors.New("failed to connect log database")
+		return errors.Wrapf(err, "failed to get log database")
 	}
 
 	requestChan := make(chan struct{})