fixed8: fix Satoshi() and add a test for it

pkg/util/fixed8.go

@@ -116,7 +116,7 @@ func (f Fixed8) MarshalJSON() ([]byte, error) {
 
 // Satoshi defines the value of a 'Satoshi'.
 func Satoshi() Fixed8 {
-	return NewFixed8(1)
+	return Fixed8(1)
 }
 
 // Div implements Fixd8 division operator.

pkg/util/fixed8_test.go

@@ -66,6 +66,11 @@ func TestFixed8DecodeString(t *testing.T) {
 	assert.Equal(t, Fixed8(90123410000), n)
 }
 
+func TestSatoshi(t *testing.T) {
+	satoshif8 := Satoshi()
+	assert.Equal(t, "0.00000001", satoshif8.String())
+}
+
 func TestFixed8UnmarshalJSON(t *testing.T) {
 	var testCases = []float64{
 		123.45,