Improve test coverage of ruleutils.c.

Improve test coverage of ruleutils.c.

While fooling around with the EXPLAIN improvements I've been working
on, I noticed that there were some large gaps in our test coverage
of ruleutils.c, according to the code coverage report.  This commit
just adds a few test cases to improve coverage of:
get_name_for_var_field()
get_update_query_targetlist_def()
isSimpleNode()
get_sublink_expr()