pgsql: Add tests for tuplesort.c.

Add tests for tuplesort.c.

Previously significant parts of tuplesort.c were untested. This
commit, while not testing every path, significantly increases
coverage.  In particular, this adds tests for abbreviated key logic,
forward/backward scans & scrolling and mark/restore.

I tried to keep the table sizes reasonable, and stress the on-disk
paths by setting work_mem to low values for specific tests. The
buildfarm will tell whether more attention to test time is needed.

Author: Andres Freund
Discussion: https://postgr.es/m/20191013144153.ooxrfglvnaocsrx2@alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4a252996d5fda7662b2afdf329a5c95be0fe3b01

Modified Files
--------------
src/test/regress/expected/tuplesort.out | 691 ++++++++++++++++++++++++++++++++
src/test/regress/parallel_schedule      |   2 +-
src/test/regress/sql/tuplesort.sql      | 304 ++++++++++++++
3 files changed, 996 insertions(+), 1 deletion(-)