parallel-tests: avoid GNU make 3.80 substitution bug.

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>