L3 Validation tests for allowedNextSiblings allowedPreviousSiblings

Late in the development of the Validation spec we agreed to change the
definition of allowedNextSiblings and allowedPreviousSiblings from
returning "all possible" siblings to the phrase "all that can be inserted".

This meant that siblings already in the instance are considered in the
decision of whether they should be returned.
It doesn't appear that the tests in the test suite have been updated to
reflect this.  Will they be updated at some point?

Patti