Add automated phpcs checks in drone for acceptance test code

[phil-davis]

The acceptance test code has been conforming to a proposed "standard" that was implemented some time ago but not formally enforced. It requires a bunch of PHP doc block standards, for example. See phpcs.xml in the root folder of the core repo.

The check is done with:

phpcs --standard=phpcs.xml tests/acceptance

Implement this much checking as a drone job, so that at least the acceptance test code stays to this standard.

Note: It is out-of-scope for this issue to decide what standard might apply to the whole code base and how to implement such a thing.

[ownclouders]

GitMate.io thinks possibly related issues are #32258 (Fix phpcs code style in acceptance tests), #31579 (test), #22485 (test), #22492 (test), and #32299 (Use --remote for acceptance tests run on drone).

[phil-davis]

PR #32785 implements this