fixed8: remove trailing zeroes from string representation

Add some testcases from `dev` branch (with less than zero case added) and
remove useless trailing zeroes from the resulting string.

pkg/util/fixed8.go

@@ -34,7 +34,7 @@ func (f Fixed8) String() string {
 		for i := len(str); i < 8; i++ {
 			buf.WriteRune('0')
 		}
-		buf.WriteString(str)
+		buf.WriteString(strings.TrimRight(str, "0"))
 	}
 	return buf.String()
 }