parallel-tests: no more spurious successes for FreeBSD make

parallel-tests: no more spurious successes for FreeBSD make

Work around a bug of FreeBSD make bug that was causing the
automake-generated "check" target to complete with success
even if some tests failed; this happened only when FreeBSD
make was run in concurrent mode (as in, e.g., "make -j2
check").  The bug is not present in NetBSD make.

This change fixes automake bug#9245:
 <http://debbugs.gnu.org/cgi/bugreport.cgi?bug=9245>

See also FreeBSD PR bin/159730:
 <http://www.freebsd.org/cgi/query-pr.cgi?pr=159730>

* lib/am/check.am [%?PARALLEL_TESTS%] $(TEST_SUITE_LOG): Use a
more "safe" (and apparently redundant) idiom to exit with error,
so that the non-zero exit status is picked up also by FreeBSD
make when it's running in concurrent mode.
* NEWS: Update.
* tests/check-concurrency-bug9245.test: New test.
* tests/Makefile.am (TESTS): Update.