Reduce duration of stats_ext regression tests

Reduce duration of stats_ext regression tests

The regression tests of extended statistics were taking a fair amount of
time, due to using fairly large data sets with a couple thousand rows.
So far this was fine, but with tests for statistics on expressions the
duration would get a bit excessive.  So reduce the size of some of the
tests that will be used to test expressions, to keep the duration under
control.  Done in a separate commit before adding the statistics on
expressions, to make it clear which estimates are expected to change.

Author: Tomas Vondra
Discussion: https://postgr.es/m/ad7891d2-e90c-b446-9fe2-7419143847d7%40enterprisedb.com