This commit:
* Removes the use of `realpath()` to prevent issues with WSL and other virtualized filesystems.
* Changes the logic of the Polyfill bootstrap loading to expect an absolute path, rather than a relative path to the root directory of the PHPUnit Polyfills library.
* Adjusts the relevant inline documentation and error messages to expect an absolute path.
* Breaks up error messages into smaller line lengths for readability.
Follow-up to [51598], [51810], [51811], [51812].
Props jrf, schlessera, hellofromTonya, jeherve, lucatume.
See #46149.
git-svn-id: https://develop.svn.wordpress.org/trunk@51813 602fd350-edb4-49c9-b593-d223f7449a82