Fixed a bug in the nestedset implementation. When using single table inheritance all node instances need to use the same tree object. previously every subclass created it's own tree object which resulted in strange behaviour.

Not using User model since users gets its table cleared before each
test method call ($this->objTable()->clear()), and that affects the
result

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deprecated validation attributes VLD, AUTO_LENGTH_VLD and AUTO_TYPE_VLD, introduced new attribute ATTR_VALIDATE

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