Extended UUID test cases to increase test coverage above 90%.

As per the title this PR increases the test coverage of uuid.go to 93% to be exact. The only code that was not tested [uuid.go#L43](https://github.com/gocql/gocql/blob/master/uuid.go#L43) and [uuid.go#L104](https://github.com/gocql/gocql/blob/master/uuid.go#L104) due to the fact that I would need to mock the system APIs to simulate the error situations.