Add "check" for all (relevant) ovn-nbctl command in unit/system tests.
Such a check is missing for some commands in some tests, causing failures of such commands to be un-noticed.
For instance the following command is called within ovn.at:
ovn-nbctl --wait=hv
If a check was added, it would have been obvious that the command was wrong (missing sync)
check ovn-nbctl --wait=hv
Note that the missing sync will be added as part of a different patch.