testing/testcase: run `slapos node report` at least 3 times

In a scenario of an instance with child instances which does not properly
support destruction, a scenario like this happens:

 1. first report detect that it's destroying a root partition but child
partitions are still started and request destruction of these child instances
 2. child partitions are destroyed
 3. root partition is destroyed

For this case we need to run 3 times