Track php errors as test failures when --filter is used

Unregistering the error handler while handling an error was confusing PHP,
this caused normal errors to be displayed in the console instead of tracked
as test failures when the --filter flag was used.

### Steps to reproduce

Add a test method that will produce any notice
```php
	public function testNotice() {
		$a = $a + 1;
	}
```

Run phpunit with the --filter flag

```shell
phpunit --filter testNotice my_test.php
```

The notice will be outputted to the console and the test will pass.