invoke: backfill tests for plugin execution

plugins/test/noop/debug/debug.go

@@ -26,6 +26,7 @@ import (
 type Debug struct {
 	ReportResult string
 	ReportError  string
+	ReportStderr string
 	Command      string
 	CmdArgs      skel.CmdArgs
 }

plugins/test/noop/main.go

@@ -51,6 +51,8 @@ func debugBehavior(args *skel.CmdArgs, command string) error {
 		return err
 	}
 
+	os.Stderr.WriteString(debug.ReportStderr)
+
 	if debug.ReportError != "" {
 		return errors.New(debug.ReportError)
 	} else {