Perf tests: send a Slack notification on performance tests failure

[adimoldovan]

Changes proposed in this Pull Request:

Send a notification on Slack when block performance tests are failing.

Jetpack product discussion

1156386383419466-as-1202291410523594/f

Does this pull request change what data or activity we track or use?

no

Testing instructions:

• See test notification: p1653484752622569-slack-C1Y1F7ZM1

[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

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[brbrr]

Looks good 👍