Symfony version(s) affected: 5.1.5

Description
This line, where DeprecationErrorHandler is being registered, relies on the fact that a SYMFONY_DEPRECATIONS_HELPER env value exists in $_ENV. Problem is, this bootstrapping happens while inside Composer autoloader:

At this point, phpunit.xml file hasn't even been loaded, as a result, the aforementioned env value which is provided there isn't put into $_ENV yet. This technically invalidates the point of this line in the code if you don't provide the env value via command shell.

Once autoloader is loaded (at this point we are past the point of loading DeprecationErrorHandler) env values are loaded from phpunit.xml:

How to reproduce

1. phpunit.xml file has line: <env name="SYMFONY_DEPRECATIONS_HELPER" value="disabled" />
2. Create some test with deprecated usage triggerable.
3. Run tests and you still will get THE ERROR HANDLER HAS CHANGED! error at the end of the test, which is emitted by DeprecationErrorHandler.
   
4. Run the same tests with env variable passed in command shell: SYMFONY_DEPRECATIONS_HELPER="disabled" phpunit and no errors will be emitted.
   

And guys... All caps?! Seriously?!