Symfony version(s) affected: 4.4.2

Description
When we update to 4.4, my tests go from this:

Time: 7.7 minutes, Memory: 295.00 MB 
 
OK (1878 tests, 8872 assertions) 

To this:

Time: 30.83 minutes, Memory: 9.03 GB

There was 1 error:

1) Hyra\Tests\Integration\Tasks\Command\WorkerCommandTest::testExecute
proc_open(): fork failed - Cannot allocate memory

vendor/symfony/console/Terminal.php:157
vendor/symfony/console/Terminal.php:143
vendor/symfony/console/Terminal.php:109
vendor/symfony/console/Terminal.php:92
vendor/symfony/console/Terminal.php:33
vendor/symfony/console/Style/SymfonyStyle.php:51
src/Tasks/Command/WorkerCommand.php:59
vendor/symfony/console/Command/Command.php:255
vendor/symfony/console/Tester/CommandTester.php:76
tests/Integration/Tasks/Command/WorkerCommandTest.php:59
 
-- 
 
There was 1 failure: 
 
1) Hyra\Tests\Functional\Negotiate\Download\DealContractFileUrlTest::testTenantsDownloadsContractWithCoverSheetAttachedInReview 
Failed asserting that 500 is identical to 200. 
 
tests/Functional/Negotiate/Download/DealContractFileUrlTest.php:118 
 
ERRORS! 
Tests: 1876, Assertions: 8852, Errors: 1, Failures: 1. 

At first, we thought that #34567 would fix it, but that was released a while back and it did not make a difference. Another possibly related issue would be #25964.

I've been unable to track down the actual issue, and can't seem to break down the problem into a reproducer on a blank Symfony installation. However, here is everything I've been able to figure out:

• Only seems to be an issue for tests extending WebTestCase and booting the kernel
• Running a single WebTestCase test uses marginally higher memory, but not by much
• Running multiple WebTestCase tests causes the memory usage to grow much faster
• Examples:
  • 1 test uses 40.5MB on 4.3 but 42.5MB on 4.4
  • 8 tests uses 48.5MB on 4.3 but 86.5MB on 4.4
  • 19 tests uses 54.5MB on 4.3 but 158.5 MB on 4.4
• Blackfire profile running a test subset on 4.3
• Blackfire profile running a test subset on 4.4
• Blackfire comparison

How to reproduce
We can't get a bug reproducer working, however these are the steps I've taken:

• composer update symfony/*
• Run tests like normal php vendor/bin/phpunit

Additional context

• To rule out problems with other package upgrades we locked Symfony to 4.3 using extra.symfony.require: "4.3.*" in composer.json then ran a composer update. Then with extra.symfony.require: "^4.2.0" ran composer update symfony/*
• Initially we was using symfony/phpunit-bridge, but ruled out an issue with that by uninstalling it and using phpunit/phpunit directly
• We're using the LiipFunctionalTestBundle and the LiipTestFixturesBundle and loading fixtures in the WebTestCase
• We are using the TestContainer to access services in tests
• The test subset I'm running does not use the PhpUnit setUp functions