…ecation testing (alexpott)

This PR was submitted for the master branch but it was squashed and merged into the 5.x branch instead.

Discussion
----------

[PhpUnitBridge] Add ability to set a baseline for deprecation testing

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes <!-- please update src/**/CHANGELOG.md files -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tickets       | Fix #37715, #34496
| License       | MIT
| Doc PR        | symfony/symfony-docs#... <!-- required for new features -->

This PR allows you set new options for `SYMFONY_DEPRECATIONS_HELPER` env var:
* `generateBaseline` - if this is set to TRUE any deprecations that occur will be written to the file defined in the `baselineFile` option
* `baselineFile` a path to a file that contains a json encoded array of deprecations that will be skipped.

### Questions
* If you set `generateBaseline` without also setting `baselineFile` an exception is thrown. We could use a default filename if one is not provided (like PHPStan).
* How much error checking should we do around the `baselineFile` variable - should we check if it is readable or should we rely on `file_get_contents`()?

### Still @todo
Add proper end-to-end testing using a .phpt test

Commits
-------

483236f [PhpUnitBridge] Add ability to set a baseline for deprecation testing