Test_vdi_cbt: add VDI.data_destroy timing tests
Test_vdi_cbt: add VDI.data_destroy timing tests

These tests slow down the unit tests a bit, but at least they test
realistic timings.

Signed-off-by: Iglói Gábor <7535f5501fd71876c93d896691e178fd3a11cfbb@citrix.com>

srowe/xen-api,srowe/xen-api,srowe/xen-api,srowe/xen-api,srowe/xen-api