Support multiple test runners as part of HHv3

[kanej]

After HH + EDR, fork main and split out the mocha test runner. We want to add an abstraction that will allow plugins to swap in an alternative test runner. A minimal alternative runner should be built based on the in-built node test runner (available since v20). A plugin that encapsulates the current mocha test runner should also be created.

It is not a goal to resolve which test runner should be default in v3. It is not a goal to determine which assertion libraries or network helpers should be used.

TODO

• [x] Pull together info on the most popular test runners, to aid our decision of default test runner
• [x] Create a test runner abstraction that is pluggable
• [ ] Move the exiting mocha runner into that abstraction
• [ ] Add a simple test runner based on the most popular test runner