Add tests for each alert

[teor2345]

Test Plan

Currently we're doing manual testing, but we should automate it (and add it to CI).

Here's how this is implemented in PR #17:

• instead of making Slack calls in each alert checker, send the found alerts through a channel to a task that sends alerts
• in tests, check known alerting blocks from the node, then check the expected alerts were sent on the channel

Later, this will allow us to combine multiple alerts from the same block into the same message, which helps with usability (and Slack rate limits).

As part of these tests, we might need to make some alert thresholds configurable.

Missing Tests

• Startup
• BlockProductionStall - use tokio::time::pause() to speed up time
• BlockProductionResumed - just supply two blocks
• ForceBalanceTransfer:
  • Failed force_transfer: https://autonomys.subscan.io/extrinsic/2173351-7
  • Failed force_set_balance: https://autonomys.subscan.io/extrinsic/1154587-11
• LargeBalanceTransfer
• SudoCall
• SudoEvent
• OperatorSlashed - we should just fake this
• SlotTime
• ChainFork/Reorg - fake these

[teor2345]

I added some notes on how to do these tests.