Remove PHPUnit and PHPCS config file path from composer scripts

[thelovekesh]

Summary

Previously: #989

Remove hard-coded config file paths to let the tools decide which config to load on the user machine. With this, the following tools will be able to:

• PHPCS will be able to prioritize phpcs.xml over phpcs.xml.dist if present locally.
• PHPUnit will be able to prioritize phpunit.xml over phpunit.xml.dist if present locally.

Relevant technical choices

Checklist

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• PR has a [Type] label.
• PR has a milestone or the no milestone label.

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