test: cli test setup & test chain head

CLI actions lack unit tests. I decided to use the approach similar to
what I found in `send_test.go` using gomock, but I don't rely on custom
"service" implementations but mock the whole FullNode API.
This first commit validates the test setup by testing the simplest method
of the chain category, e.g. `chain head`.

This requires a minor refactor of the CLI action code:
- The constructor (`GetFullNodeAPI`) checks if there's an injected mock
API in the app Metadata and uses that in unit tests.
- Actions shouldn't use raw `fmt.*` but instead write to the `app.Writer`
so the CLI output is testable

cli/util/api.go

@@ -223,6 +223,11 @@ func GetCommonAPI(ctx *cli.Context) (api.CommonNet, jsonrpc.ClientCloser, error)
 }
 
 func GetFullNodeAPI(ctx *cli.Context) (v0api.FullNode, jsonrpc.ClientCloser, error) {
+	// use the mocked API in CLI unit tests, see cli/chain_test.go for mock definition
+	if mock, ok := ctx.App.Metadata["test-full-api"]; ok {
+		return &v0api.WrapperV1Full{FullNode: mock.(v1api.FullNode)}, func() {}, nil
+	}
+
 	if tn, ok := ctx.App.Metadata["testnode-full"]; ok {
 		return &v0api.WrapperV1Full{FullNode: tn.(v1api.FullNode)}, func() {}, nil
 	}