t/jobs/t0030: add test for --bandwidth-log option

t/jobs/t0030: add test for --bandwidth-log option

Add a test to detect changes that break the --bandwidth-log option. This
option uses the logging data structures in a way that differs from how
the other logs use them. So it's easy to forget about this special case.

Recent patches resolving related breakage are:
d72244761b2230fbb2d6eaec59cdedd3ea651d4f ("stat: fix segfault with fio
option --bandwidth-log")
acc481b6d34aab3ee6e19f22b64f8bf0dd30480c ("iolog: fix reported defect
from coverity scan")

Signed-off-by: Vincent Fu <vincent.fu@samsung.com>