[llvm] [Debugify] Add 'error-test' mode for the debugify report script, for CI (PR #147574)

[Jeremy Morse via llvm-commits]

https://github.com/jmorse commented:

Looks good in general; I'm generally unfamiliar with this script though, so I'd much prefer if someone else could review it for approval.

https://github.com/llvm/llvm-project/pull/147574