Rename tables in tests of partition MERGE/SPLIT operations

Replace "salesman" with "salesperson", "salesmen" with "salespeople".  The
names are both gramatically correct and gender-neutral.

Reported-by: Alexander Lakhin
Discussion: https://postgr.es/m/fdaa003e-919c-cbc9-4f0c-e4546e96bd65%40gmail.com
Reviewed-by: Robert Haas, Pavel Borisov

Author: akorotkov