This PR introduces a new option to slogtest: IgnoreErrorFn, which allows test code to selectively ignore errors based on the contents of the SinkEntry.

primary use case:

We're still fighting flakes where the error is due to canceling the context on shut down, but when canceling active postgreSQL queries, we can get errors like: pq: canceling statement due to user request.

e.g. https://github.com/coder/coder/actions/runs/11681605747/job/32527006174

    t.go:108: 2024-11-05 09:52:37.996 [erro]  workspacestats: failed to load template schedule bumping activity, defaulting to bumping by 60min  request_id=f14215d2-73dc-47ba-aa81-422c62f257e4  workspace_id=545d73c7-3a62-4466-8c08-b6abb12867b7  template_id=49747428-3abb-40e4-a6b2-03653e9f2506 ...
        error= fetch object:
                   github.com/coder/coder/v2/coderd/database/dbauthz.(*querier).GetTemplateByID.fetch[...].func1
                       /home/runner/work/coder/coder/coderd/database/dbauthz/dbauthz.go:497
                 - pq: canceling statement due to user request
         *** slogtest: log detected at level ERROR; TEST FAILURE ***

Unfortunately, we can't use IgnoredErrorIs for this, because the exported pq.Error type encompasses all PostgreSQL errors, not just query canceled. But, if we have the freedom to use a function to ignore errors, we can use database.IsQueryCanceledError to weed out these flakes.

secondary use case:

I decided to pass the entire SinkEntry to the function, so that tests can be really targeted about which errors to ignore, if they want to. They could match on the error message, or even other fields so that you can ignore an error for a request that is expected to fail, but not mask failures for other similar requests that are supposed to succeed.