Refactor and add tests for stream filters

[Girgias]

This has uncovered a memory leak, which I'm not exactly sure how it needs to be fixed.

These tests are in preparation for some refactoring I was doing and realised there were insufficient tests.

A thing, that probably requires an RFC that would be a good idea is to mandate that the class name passed to stream_filter_register actually exists and is a child of php_user_filter

[ndossche]

Leak fix here: #20058

[ndossche]

Looks fine, I wonder whether it would be wise to add #[AllowDynamicProperties] to silence some deprecations warnings in some of these tests. We would need to add them anyway later when we reach PHP9 (or have a different solution than adding dynamic props by streams code)

[Girgias]

Looks fine, I wonder whether it would be wise to add #[AllowDynamicProperties] to silence some deprecations warnings in some of these tests. We would need to add them anyway later when we reach PHP9 (or have a different solution than adding dynamic props by streams code)

The general expectation is that a stream filter extends the built-in php_user_filter, which I think we should enforce and that class defines the properties.