Remove the option to build thread_test.c outside configure.

Remove the option to build thread_test.c outside configure.

Theoretically one could go into src/test/thread and build/run this
program there.  In practice, that hasn't worked since 96bf88d52,
and probably much longer on some platforms (likely including just
the sort of hoary leftovers where this test might be of interest).
While it wouldn't be too hard to repair the breakage, the fact that
nobody has noticed for two years shows that there is zero usefulness
in maintaining this build pathway.  Let's get rid of it and decree
that thread_test.c is *only* meant to be built/used in configure.

Given that decision, it makes sense to put thread_test.c under config/
and get rid of src/test/thread altogether, so that's what I did.

In passing, update src/test/README, which had been ignored by some
not-so-recent additions of subdirectories.

Discussion: https://postgr.es/m/227659.1603041612@sss.pgh.pa.us