Add host-tools/testrunner.sh to run the full test cycle in one step

[iarif4u]

Fixes #260. Fixes #259 (creates the host-tools/ directory; @aialvi agreed to the handover in #260).

Purpose

Hosts say that running the runner is tedious (#257). This adds a ready-made script that runs the full cycle in one step, after the runner is set up.

What host-tools/testrunner.sh does

1. Goes to the runner directory: the parent of host-tools/, or WPT_RUNNER_DIR.
2. Stops with a clear message if .env does not exist.
3. Stops if another run is still active (flock), so cron runs cannot overlap.
4. Runs git pull --ff-only origin master. If the update fails, it warns and continues. Set WPT_SKIP_UPDATE=1 to skip the update.
5. Loads .env and runs prepare.php, test.php, report.php and cleanup.php with WPT_PHP_EXECUTABLE, the same PHP that the runner uses.

Differences from the example in the issue and the README:

• The report runs when tests fail. test.php exits with 1 when any test fails. A plain set -e script would then skip the report, so failures would never be reported.
• Cleanup always runs: after a failed prepare, a failed test, or a stopped run (trap).
• Exit status is 1 when any step fails, so cron mail or monitoring can see it.

The README "Script en Bash" section now points to this script. The systemd example is not changed.

Testing

I used stub prepare/test/report/cleanup.php files that exit 0 or 1:

Case	Result
No .env	Error, exit 1
All steps pass	All 4 run, exit 0
test.php fails	Report and cleanup still run, exit 1
prepare.php fails	Test and report skipped, cleanup runs, exit 1
cleanup.php fails	Exit 1
Second run while first is active	"Another test run is still active", exit 1
SIGTERM during test	Cleanup runs
git pull fails	Warning, run continues

Contributed at WordCamp Contributor Day.

[github-actions]

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Co-authored-by: iarif4u <iarif4u@git.wordpress.org>
Co-authored-by: ramonfincken <ramon-fincken@git.wordpress.org>
Co-authored-by: aialvi <aialvi@git.wordpress.org>
Co-authored-by: kittenkamala <amykamala@git.wordpress.org>

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[ramonfincken]

!!!!
Please refrain from using our hardcode paths @iarif4u

[iarif4u]

Thanks @ramonfincken, fair point. I took /home/wptestrunner from the existing README and the #260 example, but new scripts should not suggest one host's layout. Fixed in 41575c3: the examples now use /path/to/phpunit-test-runner, and the comment notes that the script finds the runner from its own location. I made the same change in #349 (387f77b).