Improve coverage of ruleutils.c for SQLValueFunctions

Improve coverage of ruleutils.c for SQLValueFunctions

While looking at how these are handled in the parser and the executor, I
have noticed that there is no test coverage for most of these when
reverse-engineering an expression for a SQLValueFunction node in
ruleutils.c, including how these are reparsed when included in a FROM
clause.  Some hacking in this area has showed me that these could break
easily, so add some coverage to track the existing compatibility.

Extracted from a much larger patch by me.

Discussion: https://postgr.es/m/YzaG3MoryCguUOym@paquier.xyz