check: new developer-reserved AM_TESTS_SETUP variable

check: new developer-reserved AM_TESTS_SETUP variable

For reference, see the discussion at:
<http://lists.gnu.org/archive/html/automake-patches/2011-01/msg00213.html>

* lib/am/check.am [%?PARALLEL_TESTS%] (am__check_pre): Pass also
$(AM_TESTS_SETUP).  Comments updated, and some typos fixed.
* doc/automake.texi (Simple Tests using parallel-tests): Document
AM_TESTS_SETUP.  Reorder some of the existing documentation a bit.
* tests/parallel-tests-am_tests_setup.test: New test.

From a suggestion by Ralf Wildenhues.