Re: pgsql: Drop test user when done with it.

I wrote:
> Michael Paquier <michael@paquier.xyz> writes:
>> If we are on that, we still have src/test/modules/test_pg_dump/ which
>> is not repeatable with multiple installchecks:
>> https://www.postgresql.org/message-id/20181130163728.GE3415@tamriel.snowman.net

> OK, hadn't run into that personally, but let's fix that too.  Anything
> that can be run with "installcheck" has to satisfy the restrictions
> of being re-runnable and careful about what global names it uses.

Actually, now that I re-read that thread, maybe what we need to do is
mark test_pg_dump as something not to be run by "make installcheck"?
I'm not quite sure what *does* run it, but the context is evidently
that it's supposed to be run by some overarching script that's then
going to run pg_dump on the ending database state.  So it's not meant
to be run against a generic pre-existing installation, and it doesn't
have to follow the rules for being safe for that --- but then we need
to be sure that it doesn't get called that way if someone does "make
installcheck" in a parent directory.