fix(docker): add default CMD to test stage in Dockerfile #1499

Job	Run time
Unit tests / Test all	0s
Build CI Docker / Build images	0s
Unit tests / Test checkpoint sync from empty state	0s
Unit tests / Test with fake activation heights	0s
Integration tests / Check if cached state disks exist for Mainnet / Get Mainnet cached disk	0s
Unit tests / Test CI default Docker config file / Test default-conf in Docker	0s
Unit tests / Test integration with lightwalletd	0s
Unit tests / Test CI custom Docker config file / Test custom-conf in Docker	0s
Integration tests / Generate checkpoints mainnet / Run checkpoints-mainnet test	0s
Integration tests / Zebra tip JSON-RPC / Run fully-synced-rpc test	0s
Integration tests / Zebra checkpoint update / Run sync-past-checkpoint test	0s
Integration tests / Zebra tip update / Run update-to-tip test	0s
Integration tests / get block template / Run get-block-template test	0s
Integration tests / lightwalletd tip / Run lwd-full-sync test	0s
Integration tests / lightwalletd GRPC tests / Run lwd-grpc-wallet test	0s
Integration tests / lightwalletd tip send / Run lwd-send-transactions test	0s
Integration tests / lightwalletd tip update / Run lwd-update-sync test	0s
Integration tests / submit block / Run submit-block test	0s
Unit tests / Test CI testnet Docker config file / Test testnet-conf in Docker	0s
	0s

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