[Process] Fix #8739

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8739
| License       | MIT

This adds a fix to #8739. Whenever a call is done to to any non-blocking methods (`Process::isRunning`, `Process::isStopped`, `Process::isTerminated`, `Process::getStatus`, `Process::getPid`...), buffers are read, and callback executed.

Such code will now work : 

```
$process->start(function ($type, $data) {
    echo $data;
});

while ($process->isRunning()) {
    // some stuff
    // callback is executed
}
```