At the moment, Thread.abort_on_exception=true regardless of whether you have Safely or not. I think I agree with this, however this is done in the configure_safely method. I think you should rename this method to something else, e.g. configure_exception_handling, and then extract the safely stuff to configure_safely. You can then invoke that method if Safely is defined, ala configure_safely if safely_enabled?. The safely_enabled? method can be stubbed under test to return false for the case where you want to assert that it won't be configured when the gem isn't available. This is better than fiddling with constants at test time.
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