[FAB-4349] Improve UT coverage of orderer/solo
[FAB-4349] Improve UT coverage of orderer/solo

Change-Id: I45ae124305f720e552a47403c518f46623bd194e
Signed-off-by: Jay Guo <91d7eeb83d08be0dc56ea674009c4b53383e15b3@gmail.com>

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