QA: Decorate tests with instance checks

Start using “InstanceCheck” decorator where appropriate. Use direct call
for check in places where the instance status changes during the test.

The parameters of “TestInstanceImport” had to be re-ordered to have the
instance as the first argument.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>