simple tests: support developer-defined fd redirections

simple tests: support developer-defined fd redirections

Motivated by coreutils bug#8846, and related discussions:
 <http://debbugs.gnu.org/cgi/bugreport.cgi?bug=8846>
 <http://lists.gnu.org/archive/html/bug-autoconf/2011-06/msg00002.html>
In those threads it has been shown how problematic it can be to do
portable file descriptor redirections in the testsuite when using
the Automake testsuite harness.  This change should remedy to that
situation.

* lib/am/check2.am (?GENERIC?%EXT%$(EXEEXT).log,
?GENERIC?%EXT%.log, ?!GENERIC?%OBJ%): Append redirection defined
in $(AM_TESTS_FD_REDIRECT) to the command-line invocations of the
test scripts.
* lib/am/check.am [!%?PARALLEL_TESTS%] $(check-TESTS): Likewise.
* NEWS: Update.
* doc/automake.texi (Script-based Testsuites): Document the new
feature.
* tests/check-fd-redirect.test: New test.
* tests/parallel-tests-fd-redirect.test: Likewise.
* tests/parallel-tests-am_tests_environment.test: Remove checks
about the use of redirections in AM_TESTS_ENVIRONMENT: they would
check deprecated (if not undefined) behaviour now.  Strengthen a
couple of still valid checks, to keep the test more in sync with
the documentation.  Improve debugging information.
* tests/Makefile.am (TESTS): Update.