Stabilize results of brin regression test.

Stabilize results of brin regression test.

This test used seqscans on tenk1, with LIMIT, to build test data.
That works most of the time, but if the synchronized-seqscan logic
kicks in, we get varying test data.  This seems likely to explain
the erratic test failures on buildfarm member chipmunk, which uses
smaller-than-default shared_buffers.  To fix, add ORDER BY clauses to
force the ordering to be what it was implicitly being assumed to be.

Peter Geoghegan had noticed this with respect to one of the trouble
spots, though not the ones actually causing the chipmunk issue.