E2E_Core_HookTest - Fix test failure due to leak

Before
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The `HookTest` class fails when running on WordPress.

The event-listener from `testSymfonyListener_names` leaks and remains active during `testSymfonyListener_int`.

After
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The `HookTest` class passes when running on WordPress.

The event-listeners from `testSymfonyListener_names` and `testSymfonyListener_int` are cleaned up.