Add shortcuts to simplify running PHPUnit tests for standalone plugins

[felixarntz]

Summary

See #946 (comment): This PR adds shortcuts to simplify running PHPUnit tests for the new standalone plugins in a way that the new plugins only have to be added to composer.json and are then easy to test.

The way the shortcuts are defined is heavily inspired by #1002.

Usage

• composer test:auto-sizes: Tests the Auto Sizes plugin on single site.
• composer test-multisite:auto-sizes: Tests the Auto Sizes plugin on multisite.
• composer test:plugins: Tests all standalone plugins on single site.
• composer test-multisite:plugins: Tests all standalone plugins on multisite.
• composer test:all: Tests all standalone plugins plus Performance Lab on single site.
• composer test-multisite:all: Tests all standalone plugins plus Performance Lab on multisite.

There's also an NPM shortcut specifically for the GitHub workflow to test all standalone plugins:

• npm run test-php-plugins: Wraps composer test:plugins.
• npm run test-php-multisite-plugins: Wraps composer test-multisite:plugins.

Checklist

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

[github-actions]

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Co-authored-by: felixarntz <flixos90@git.wordpress.org>
Co-authored-by: westonruter <westonruter@git.wordpress.org>
Co-authored-by: swissspidy <swissspidy@git.wordpress.org>

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[swissspidy]

LGTM but long term we should find something more scalable for this kind of scripts when we have more plugins.