Create a separate test file for exercising system views

In connection with the "pg_hba_file_settings view patch" thread, I was
wondering where we could logically insert a regression test case for that
view. I realized that there is no natural home for it among the existing
regression tests, because it's not really connected to any SQL language
feature. The same is true for a number of other built-in views, and
unsurprisingly, most of them are not exercised anywhere :-(.

Accordingly, I propose creating a new regression test file whose charter
is to exercise the SRFs underlying system views, as per attached.
I don't think we desperately need new tests for views that expand to
simple SQL, but these test cases correspond directly to code coverage
for C functions, so they seem worthwhile.

I did not do anything about testing the various pg_stat_xxx views.
Those could be added later, or maybe they deserve their own home.
(In many cases, those would need something smarter than the basic
count(*) technique used here, because the C functions are invoked
in the view's SELECT list not in FROM, so the planner would throw
away those calls.)

Comments/objections?

regards, tom lane