Force the use of the old serial test harness in automake

In automake 1.13 the default way of running tests has changed to using the
"parallel test harness". This is supposed to run tests in parallel and
automate the creation of log files.

However:

1) It does not appear to work with recursive makefiles, so using it would
   require a complete rewrite of the self test makefiles.

2) it breaks the "TESTS_ENVIRONMENT=true" hack (which allows building of
   tests without running them). Which is very handy in general and required
   for the parallel_self_tests.py script.

So for now I have forced to use the old test harness. It may be better to
use the new one in the future if we can rearrange our
makefiles. Alternatively it may be better to drop the use of automake's
test harness altogether since recursively running validate.sh scripts could
easily be done by a shell script.
Make fpdiff and parallel_self_tests work when python3 is default

* fpdiff still uses python2, but now this is explicitly specified by
  the hashbang line.

* parallel_self_tests should work with any version