pgsql: 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()

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/830d1c73b3f4524bc897ddab5c6c3b47840c915a

Modified Files
--------------
src/test/regress/expected/create_view.out | 103 +++++++++++++++++++++++++++++-
src/test/regress/expected/rules.out | 24 +++++++
src/test/regress/sql/create_view.sql | 35 ++++++++++
src/test/regress/sql/rules.sql | 11 ++++
4 files changed, 172 insertions(+), 1 deletion(-)