text coverage for EXTRACT()

During the discussion in [0] I noticed that the extract()/date_part() 
variants for time, timetz, and interval had virtually no test coverage. 
So I put some more tests together, which should be useful if we decide 
to make any changes in this area per [0].


[0]: 
https://www.postgresql.org/message-id/flat/42b73d2d-da12-ba9f-570a-420e0cce19d9%40phystech.edu

-- 
Peter Eisentraut              http://www.2ndQuadrant.com/
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