shared.control: update bonnie control file

autotest.bonnie always failed because of not enough free space to
create the testing file on a 20G virtual disk.

Therefore, update default test command to:

"bonnie++ -d xxx -n 500 -s 2048 -r 1024"

to avoid this situation.

Signed-off-by: Xu Tian <xutian@redhat.com>