Add docs/testing.md covering the dev_cli.sh interface, all
test types (unit, integration, VFIO, Windows, live migration,
rate limiter, CVM), custom kernel/firmware overrides via
environment variables, performance metrics, code coverage,
and the CI workflow matrix.
Assisted-by: GitHub Copilot:Claude-Opus-4.6
Signed-off-by: Muminul Islam <muislam@microsoft.com>
