t/zbd: make the test script easier to terminate

t/zbd: make the test script easier to terminate

Very often, it takes more than one ^C to terminate test-zbd-support
script. Just a single ^C does end the test that is currently being
executed, but then the script proceeds to the next test. This commit
adds a simple signal handler to exit the test loop after receiving
a Ctrl-C.

Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Dmitry Fomichev <dmitry.fomichev@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>