test(blade): pin that a discovery request reaches the handler

Add a unit test for the entry decision of the blade preprocessor. The test
lifts the entry out of the plugin source and drives it, because the plugin
needs a management node to load in full.

The test gives the entry the request that a booting node sends, which names
no node, and shows that the entry hands it on and answers no error. It also
shows that a request for another command without a noderange is still
refused.