EXTRACT of date type is implemented as a wrapper around EXTRACT of
timestamp, so the code is already tested there.  But the externally
visible behavior of EXTRACT on date is not recorded anywhere.  Since
there is some discussion about reimplementing or refactoring some of
this, add some more explicit tests of EXTRACT on date, similar in
structure to existing EXTRACT tests on other data types.

Discussion: https://www.postgresql.org/message-id/flat/42b73d2d-da12-ba9f-570a-420e0cce19d9@phystech.edu