feat: grind test

[charlielye]

Adds ability to trigger a test grind from the dashboard by clicking icon next to flaked/failed test.

• A given test cmd can be grinded once per 24 hours.
• By default it's ground for 10m (todo: Make manually configurable).
• Runs are under new deflake dashboard.
• bb formatter only prints to stdout if it actually has something to format.
• Publish failed ci runs to redis for potential future handling.
• run_test_cmd quite heavily refactored to cleanup many conditionals into config flags, and separate out test run logic into exec_test script.
• When we run with the Makefile flow (re-enabled on x3 and x4 merge queue runs), we inject MAKEFILE_TARGET into the harness params. This allows targetted rebuilds for grinding (and bisecting, to come later).
• Refactor slack notification stuff into slack_notify script.
• Extend local log expiry to 2 days (from 8 hours).
• New squash_args script which can help when dealing with feeding complex commands to e.g. denoise. Allows passing as separate args e.g. squash_args denoise my_cmd arg1 "arg2.1 arg2.2" and translates to denoise "my_cmd arg1 arg2.1\ arg2.2". This is really a stop gap to making scripts like denoise handle multiple args. We made it accept a single quoted arg so we could print it easily, but it creates quote nightmares.
• Adds grind-test cmd to ci.sh (and bootstrap).
• Start the makefile test_engine with setsid. Think I was wrong about using ()& as that doesn't give a new process group when non-interactive.

[ludamad]

Ah, elegant way to add a knob here