"make check" in src/test/isolation is unworkable

I believe that the "make check" target in src/test/isolation is
fundamentally unportable, as is illustrated by the fact that buildfarm
member coypu is currently choking on it. The reason is that the
pg_isolation_regress program depends on libpq, and in particular it
depends on having an *installed* libpq. Anyplace where it appears to
work, it's because you already installed Postgres, or at least libpq.

Apparently coypu is the only buildfarm member that hasn't got a
reasonably recent libpq already installed in system directories; or
maybe it's just the first such that's tried to run the isolation-test
script step.

While we could maybe hack this to the point where it works (on some
platforms) by dynamically linking libpq from the source tree, I don't
think it's worth the trouble.

Recommendations:

1. Modify the buildfarm script to run "make installcheck" in the
isolation-test step, and of course move that to after doing the install
step.

2. Get rid of the "check" target in src/test/isolation/Makefile.
We don't need to be dealing with bug reports from people who try to
use it and get either a link failure (easily diagnosed) or a libpq
version compatibility problem (not so easily diagnosed).

regards, tom lane