test/zdtm: add option -n for batch test

In batch-test mode, the script skips failed individual tests
instead of exiting. This ensures that all zdtm tests can be run
in one pass, which provides better overview of test result.

Signed-off-by: Yicheng Qin <yichengq@google.com>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>