cargo nextest is defacto the new standard test runner, and offers many improvements compared to the standard test runner (per-test-process-isolation, junit support, built-in flake handling).

Steps:

• Add cargo-nextest to bootstrap.
• Update ./mach test-unit to use cargo nextest instead of cargo test
• Check if there are any unit-tests which rely on memory-sharing / in-process serialization via a Mutex and adapt those
• Update unit-test CI to use cargo-nextest builtin retry handling instead of the retry github action
• Modify custom test harness of API tests to conform to nextest custom harness expectations