[Feature Request]: Append parameter in loadAllFixtures method

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Hello!

I commonly set the `append` parameter to true when loading fixtures via `loadFixtures`.

I recently started splitting my fixtures into many small classes, and now I would rather use `loadAllFixtures` to avoid having to write them all down in an array. However, I see that the `append` option is not present in `loadAllFixtures`. Is there a reason for this? It would seem very useful to me. Let me know if my use-case is weird or I am doing something wrong.

Thanks in advance!

### Describe alternatives you've considered

Of course, one could simply use `loadFixtures` listing all the existing fixtures as an array. It just seems inconvinient.

### Additional context

```
abstract public function loadFixtures(array $classNames = [], bool $append = false): AbstractExecutor;
public function loadAllFixtures(array $groups = []): ?AbstractExecutor
```

Start at the loadAllFixtures and loadFixtures entry points and compare their current implementations and tests. Verify how the append option behaves for loadFixtures, then confirm that loadAllFixtures accepts and forwards the option without breaking existing fixture-loading behavior.

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