Fix TestAccMonitoringGroup_monitoringGroupSubgroupExample test

[rileykarson]

This is an intermittent issue where destroy appears to fail. It appears to fail 1/5 times, so not quite enough to register as a flake.

[rileykarson]

Fixing this is probably "make a best effort attempt and we see if it comes up later" more than guaranteeing a fix.

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

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