wallet: also assert no error

wallet/wallet_test.go

@@ -11,7 +11,8 @@ func TestWallet(t *testing.T) {
 
 	t.Run("withdraw", func(t *testing.T) {
 		wallet := Wallet{balance: Bitcoin(20)}
-		wallet.Withdraw(10)
+		err := wallet.Withdraw(10)
+		assertNoError(t, err)
 		assertBalance(t, wallet, Bitcoin(10))
 	})
 
@@ -44,3 +45,11 @@ func assertError(t testing.TB, got error, want error) {
 		t.Errorf("got %q, want %q", got, want)
 	}
 }
+
+func assertNoError(t testing.TB, got error) {
+	t.Helper()
+
+	if got != nil {
+		t.Fatal("get an error but didn't want one")
+	}
+}