Fix parallel-tests with empty $(TESTS) for BSD make. * lib/am/check.am [PARALLEL_TESTS] ($(TEST_SUITE_LOG)): Ignore empty line stemming from empty $(TEST_LOGS), to avoid counting zero tests as one. (check-TESTS): If `$(TESTS)' is empty, override TEST_LOGS to be empty, rather than `.log', for BSD make. * tests/parallel-tests6.test: New test. * tests/Makefile.am: Update. Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>