Create regress_tblspc in test_setup

Create regress_tblspc in test_setup

An upcoming test needs to use a tablespace as part of its test. Historically,
we wanted tablespace creation be done in a dedicated file, so it's easy to
disable when testing replication. But that is not necessary anymore, due to
allow_in_place_tablespaces.

Create regress_tblspace tablespace in test_setup. Move the tablespace test to
the end of the parallel schedule, so other tests can use it.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20200124195226.lth52iydq2n2uilq@alap3.anarazel.de