buildfarm does not test "make check"

So I added the brin isolation test back. Because it needs the
pageinspect module, it can only work under "make check", not
installcheck. The problem is that this means buildfarm will not run it,
because it only runs installcheck :-(

I realized that the important modules that run under "make check"
already have custom buildfarm modules, namely pg_upgrade and
test_decoding. I wonder if it would make more sense to have this be
customizable from the buildfarm server side, without having to resort to
writing a buildfarm module for each new thing. I envision a file hosted
in the buildfarm server, perhaps a JSON blob, that lists modules that
need "make check" treatment. Within the JSON object for each such
module there could live the details such as what files need to be
saved. That way, we could add more things to test without requiring the
buildfarm client to be upgraded each time.

This approach seems to have worked very well to customize the branches
that each animal builds, which is why I suggest that it be used here
too.

--
Álvaro Herrera 33.5S 70.5W
"The Gord often wonders why people threaten never to come back after they've
been told never to return" (www.actsofgord.com)