test_runner: add 'test:summary' event
This commit adds a new 'test:summary' event to the test runner's reporting interface. This new event serves two purposes:
- In the future, the test runner internals will no longer need to change the process exit code. This may be important to run() users. Unfortunately, this is a breaking change, so it needs to be changed in a major version.
- The reporting interface now has a single event that can identify passing or failing test runs.
Refs: https://github.com/nodejs/node/issues/53867 Refs: https://github.com/nodejs/node/issues/54812
(Still need to add docs)