Support FK tests on temp tables

[decibel]

If you're testing code that deals with table structure you might need to create temporary objects, and the easiest way to do that is to put them in pg_temp. The existing fk_ok() failed in that case because it only recognized the numbered temp schema (ie: pg_temp_2).

This adds special support for when the user specifies pg_temp as one of the schemas.

BTW, you might want to merge this as a squash commit.

[theory]

Should pg_all_foreign_keys be updated to show temp tables, instead, perhaps with a boolean column to indicate if a table is a temp table?

[decibel]

I don't think it'd help much, since fk_ok would still need to special-case 'pg_temp'.