Add PHPUnit test runner automation example

[ekamran]

Summary

• Adds an example automation script for the PHPUnit test runner.
• Shows the expected prepare, test, report, and cleanup sequence.
• Adds a lock to avoid overlapping scheduled runs and includes a cron example.

Testing

• Ran git diff --check.
• Verified the shell example with bash -n.
• Checked the flow against the current WordPress/phpunit-test-runner README and .env.default.

[vairafiq]

Really helpful to have a concrete end to end runner script. The use of flock to prevent overlapping runs, set -u, git pull with fast forward only, propagating the first non zero exit status across report.php and cleanup.php, chmod 700, and the cron example make this a good production ready reference. The terminology cleanups (trunk instead of master, and GitHub casing) are welcome too. One optional thought: the script sources .env and references paths like /home/wptestrunner/, so a short note that these must match the values set earlier in the guide could help first timers, though it is not blocking. LGTM.