LU-11519 hsm: improve the testing of hsm.max_requests

LU-11519 hsm: improve the testing of hsm.max_requests

Modify sanity-hsm::test_250() to send more diverse HSM request.

This brings better code coverage (it would have caught the bug
reported in LU-11519).

Test-Parameters: trivial testlist=sanity-hsm,sanity-hsm,sanity-hsm
Signed-off-by: Quentin Bouget <quentin.bouget@cea.fr>
Change-Id: I0142b46dc804f649c33deb81efea7f68d1e29afa
Reviewed-on: https://review.whamcloud.com/33590
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Ben Evans <bevans@cray.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>