Tests: add end-to-end tests for all supported CLI arguments

[jrfnl]

End to end tests should be introduced in a separate test suite to cover the use of all supported CLI arguments (and some combinations of these).

PHPUnit supports a phpt format to run end to end tests, though there is not much documentation available on writing them (see sebastianbergmann/phpunit-documentation-english#302).
Alternatively, something like the expect could possibly be used.

I've written some tests as a proof of concept and will try to make those publicly available in the near future to serve as an example to expand on.

The end-to-end tests should preferably be run in CI on:

• Different OSes (Linux, Mac, Windows)
• A few different PHP versions (high/low supported versions and possible some extra if there are specific issues which need to be tested on specific versions)
• Using the PHAR file + using a Composer install
• With a few different extensions enabled/disabled combinations, including PCNTL and gRPC (see PHP gRPC extension breaks use of --parallel #294).
• With a few different combinations of relevant ini settings (can likely be configured per test)