CI: add a test run against PHPCS 4.x (when it is closer to being released)

Follow up action after #628

~~Note: as things stand, testing against PHPCS 4.x requires to run `composer install` using `--prefer-source` as the abstract test case file is no longer included in the package for distribution. There is an issue open upstream to discuss this further.~~
~~See: https://github.com/squizlabs/PHP_CodeSniffer/issues/3158~~

Loosely related to #659 too.

Review the follow-up context in #628 and the loosely related #659, then inspect the repository's existing CI setup and the current PHPCS dependency configuration. Confirm when PHPCS 4.x is close enough to test, account for the noted composer install requirement, and consider the work done when CI includes a passing PHPCS 4.x run.

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