Add test coverage for EXTRACT()

Add test coverage for EXTRACT()

The variants for time and timetz had zero test coverage, the variant
for interval only very little.  This adds practically full coverage
for those functions.

Reviewed-by: Vik Fearing <vik@postgresfriends.org>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/c3306ac7-fcae-a1b8-1e30-6a379d605bcb%402ndquadrant.com