High memory usage growing with each consecutive test

[arrilot]

Steps to reproduce:

1. Install basic yii2 template
2. Add tests https://github.com/arrilot/yii2-codeception-memory-leak-demo/pull/1/files
3. run vendor/bin/codecept run tests/unit/leak/A500Test.php

Time: 00:02.015, Memory: 156.00 MB
OK (500 tests, 500 assertions)

4. run vendor/bin/codecept run tests/unit/leak/A1000Test.php

Time: 00:06.381, Memory: 300.00 MB
OK (1000 tests, 1000 assertions)

1000 tests consumed twice as much memory as 500 tests. Module is not freeing 300kb of memory after each iteration of one test

5. run both test files - vendor/bin/codecept run tests/unit/leak/

Time: 00:12.377, Memory: 446.00 MB
OK (1500 tests, 1500 assertions)

Module is not freeing 300kb of memory after each test even after all tests in a file were finished

6. comment Yii2 module out in unit.suite.yml

Time: 00:00.349, Memory: 20.00 MB
OK (1500 tests, 1500 assertions)

Memory usage is 20 smaller

phpinfo.txt