Re: tweak to a few index tests to hits ambuildempty() routine.

I wrote:
> Yeah.  You can see that the coverage-test animal is not reaching it
> anymore:
> https://coverage.postgresql.org/src/backend/access/gin/ginvacuum.c.gcov.html

That's what it's saying *now*, but after rereading this whole thread
I see that it apparently said something different last week.  So the
coverage is probabilistic, which squares with this discussion and
with some tests I just did locally.  That's not good.  I shudder to
imagine how much time somebody might waste trying to locate a bug
in this area, if a test failure appears and disappears regardless
of code changes they make while chasing it.

I propose that we revert 4fb5c794e and instead add separate test
cases that just create unlogged indexes (I guess they don't actually
need to *do* anything with them?).  Looks like dec8ad367 could be
reverted as well, in view of 2f2e24d90.

            regards, tom lane