wallet: implement stringer interface

wallet/wallet.go

@@ -1,7 +1,13 @@
 package wallet
 
+import "fmt"
+
 type Bitcoin int
 
+func (b Bitcoin) String() string {
+	return fmt.Sprintf("%d BTC", b)
+}
+
 type Wallet struct {
 	balance Bitcoin
 }

wallet/wallet_test.go

@@ -11,6 +11,6 @@ func TestWallet(t *testing.T) {
 	want := Bitcoin(10)
 
 	if got != want {
-		t.Errorf("got %d want %d", got, want)
+		t.Errorf("got %q want %q", got, want)
 	}
 }