The user module tests didn't pass before this review for several reasons. After review the tests now passes all except for default timezone check.
$this->assertEqual($user->timezone, variable_get('date_default_timezone', NULL), t('Correct timezone field.'));
After a look at the code this appears to be a bug.
Other than that this test has been cleaned up quiet a bit. I removed the access rules test since that is no longer in core.
As a note
UserValidationTestCase is full of unit tests. Once unit testing is implemented they should be moved.