Try to stabilize EXPLAIN output in partition_check test.

Try to stabilize EXPLAIN output in partition_check test.

Commit 7d8ac9814bc9bb6df2d845dbabed38d7284c7c2c adjusted these
tests in the hope of preserving the plan shape, but I failed to
notice that the three partitions were, on my local machine, choosing
two different plan shapes.  This is probably related to the fact
that all three tables have exactly the same row count.  Try to
improve the situation by making pht1_e about half as large as
the other two.

Per Tom Lane and the buildfarm.

Discussion: http://postgr.es/m/25380.1519277713@sss.pgh.pa.us