Add support for stress tests to nextest

Add support to run a single test multiple times in parallel. This is a somewhat different mode of operation from nextest as usual (the runner should be fed the same test multiple times, and should maybe not run other tests at the same time), but is worth doing as a future improvement.

No files or tests are named. Start by tracing nextest's test-runner entry point and how it schedules individual tests, then determine how a stress mode would feed one test repeatedly without other tests running concurrently. Done means a single test can be run multiple times in parallel with its behavior and scheduling defined.

Written by the indexing model from the issue text.