pgsql: Add regression test case exercising the sorting path for hash in

Add regression test case exercising the sorting path for hash index build.

We've broken this code path at least twice in the past, so it's prudent
to have a test case that covers it.  To allow exercising the code path
without creating a very large (and slow to run) test case, redefine the
sort threshold to be bounded by maintenance_work_mem as well as the number
of available buffers.  While at it, fix an ancient oversight that when
building a temp index, the number of available buffers is not NBuffers but
NLocBuffer.  Also, if assertions are enabled, apply a direct test that the
sort actually does return the tuples in the expected order.

Peter Geoghegan

Patch: <CAM3SWZTBAo4hjbBd780+MrOKiKp_TMo1N3A0Rw9_im8gbD7fQA@mail.gmail.com>

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/9563d5b5e4c75e676d73a45546bd47b77c2bd739

Modified Files
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src/backend/access/hash/hash.c             | 20 +++++++++++++++++---
src/backend/access/hash/hashsort.c         | 23 ++++++++++++++++++++---
src/test/regress/expected/create_index.out |  7 +++++++
src/test/regress/sql/create_index.sql      |  7 +++++++
4 files changed, 51 insertions(+), 6 deletions(-)