[PhpUnitBridge] Enable Prophecy integration via env variable

[derrabus]

Description
PHPUnit's built-in Prophecy integration is deprecated and will be removed soon. The replacement package phpspec/prophecy-phpunit however only works with PHPUnit 9.

Maybe we can have simple-phpunit install that package if we're running PHPUnit 9 and a yet to be defined environment variable is set.

[nicolas-grekas]

Like SYMFONY_PHPUNIT_REQUIRE to mirror SYMFONY_PHPUNIT_REMOVE?

[derrabus]

Yes something like that. But with the extra constraint that phpspec/prophecy-phpunit will only be installed for PHPUnit 9.1 and above and assumed to be present if we're on a lower PHPUnit version.

[nicolas-grekas]

We do remove prophecy by default in the bridge...
I'd better not do any fancy logic here...

[acasademont]

This would be great! Right now we have a ton of deprecation messages when using phpunit 9.4 provided by the bridge that we can't really fix.

PS: Unless we require phpspec/prophecy-phpunit directly on our composer, which is not really desirable as it will install phpunit as a dependency