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>