Allow scripts/tests to work with non-standard code structure

Problem/Motivation

This is a follow up from 🐛 Wrong DRUPAL_ROOT with non-standard code structure Needs review . For security reasons, site-builders might prefer to locate scripts outside of the application root. Currently this doesn't work because Drupal guesses the app root by running __DIR__. This applies to

1. All test scripts
2. /core/scripts/*
3. /core/lib/Drupal/Core/Command/*

Proposed resolution

Possible ideas:

1. Environment variable to set app root.
2. Composer plug-in "drupal-scaffold" has a setting for "web-root" which typically has the value "web/". So this plug-in at least in theory knows the app root and could perhaps pass it in.

Remaining tasks

User interface changes

None

API changes

None

Data model changes

None

Release notes snippet

None