Fix table name clash in recently introduced test

Fix table name clash in recently introduced test

A few buildfarm animals recently started complaining about the "child"
relation already existing. e056c557aef added a new child table to inherit.sql,
but triggers.sql, running in the same parallel group, also uses a child table.

Rename the new table to inh_child. It maybe worth renaming child, parent in
other tests as well, but that's work for another day.

Discussion: https://postgr.es/m/20230407204530.52q3v5cu5x6dj676@awork3.anarazel.de