osd: tests for osd bench

Check that the safeguards against timeouts work as expected and that the
default hardcoded values are valid.

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #2011 from dachary/wip-osd-bench

osd: tests for osd bench

Reviewed-by: Huang Jun <hjwsm1989@gmail.com>