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/chain.go

@@ -67,6 +67,8 @@ var ChainHeadCmd = &cli.Command{
 	Name:  "head",
 	Usage: "Print chain head",
 	Action: func(cctx *cli.Context) error {
+		afmt := NewAppFmt(cctx.App)
+
 		api, closer, err := GetFullNodeAPI(cctx)
 		if err != nil {
 			return err
@@ -80,7 +82,7 @@ var ChainHeadCmd = &cli.Command{
 		}
 
 		for _, c := range head.Cids() {
-			fmt.Println(c)
+			afmt.Println(c)
 		}
 		return nil
 	},