parallel-tests: avoid GNU make 3.80 substitution bug. * lib/am/check.am [PARALLEL_TESTS] (check-TESTS): Remove any `.log' entries from `$(TEST_LOGS)' even if the list is nonempty, to work around GNU make 3.80 substitution reference issue with trailing white space in the variable. * tests/parallel-tests10.test: New test. * tests/parallel-tests6.test: Update comment. * tests/Makefile.am: Update. * NEWS: Update. Report by Bob Friesenhahn. Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>