tests, imsm: Set new_num_disks value corectly to perform expected size calculations

tests, imsm: Set new_num_disks value corectly to perform expected size calculations

In some migration tests, variable new_num_disks should be set to expected
number of disks after migration. This is required for proper expected size
calculation.

Pass new_num_disks variable during test execution for:
- 16imsm-r0_3d-migrate-r5_4d
- 18imsm-r1_2d-takeover-r0_1d
- 16imsm-r0_5d-migrate-r5_6d

Signed-off-by: Michal Zylowski <michal.zylowski@intel.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>