Simplify the logging in pekko CI tests

The pullRequest CI workflows `sbt validatePullRequest` step logs a large amount of information to stdout (over 15k) lines such that it is very difficult to identify where the source of a failure is.

For example:

https://github.com/apache/incubator-pekko/actions/runs/4818338277

There are multiple `Error` messages, though some of these are for tests that want to generate errors.

The actual error is found by searching the logs for `** FAIL`.

Can we improve observability here by reducing the amount of solution logs that are output to CI terminal? We could extract full logs to a cache file. When a failure is encountered for a specific test, the log output should show : , see logs in .txt for more information.

Obviously we want to take care not to affect solution logging here.