New parallel regression test shell and related things.

src/backend/access/transam/xlog.c

src/backend/utils/cache/catcache.c

src/test/regress/GNUmakefile

src/test/regress/expected/numeric.out

src/test/regress/regress.sh

src/test/regress/run_check.sh

src/test/regress/sql/numeric.sql

src/test/regress/sql/run_check.tests