[Feature] Logging output assertion

[ysugimoto]

Kind of proposals

You can turn on multiple checkboxes.

• [] Parser
• [] Linter
• [] Simulator
• Testing

Describe the feature

We need a new log message assertion function.
To assert what log message is outputted inside VCL runtime, need testing function which retrieves log statement output.

// @scope: RECV
// @suite: logging assertion
sub test_recv {
  testing.call_subroutine("vcl_recv");
  assert.match(testing.logged_message(), "^\[log]:\s.*");
}

Give us your considerations

We should consider about realtime loggings that will send to the S3, Datadog, etc.

[richardmarshall]

Not sure if you've already began work on this but I got started on something similar to this a while back but got distracted and didn't finish it up for a PR.

https://github.com/richardmarshall/falco/commits/testing_getlog/

The primary motivation for what I was doing was to enable validating structured JSON log messages with a assert.is_json test function. If you're already working on this I can just rebase that work on what you've done. If not I'm happy to clean up the whole branch and submit that as a fix for this issue.