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