Allow to test a check config

[fclairamb]

We should consider being able to actually test a check, this means probably sending it to the actual worker it's targeting in rela-time and then limiting the use of this feature. This would provide a way for humans and more importantly LLMs to immediately test if their current config is ok.
For basic HTTP config this is not needed, but for advanced javascript checks this would allow to save a lot of time.
Still, this probably require a rework of the deported agents because they currently don't handle immediate execution (workers do but this might not be useable as-is either).