server_process_unittest.py's MockFile should all be bytes

[Sam Sneddon [:gsnedders]]

This can't happen under our current testing setup, but playing with pytest locally on Python 3 I hit:

                if out_fd in read_fds:
                    data = self._proc.stdout.read()
                    if not data and not stopping and (self._treat_no_data_as_crash or self._proc.poll()):
                        _log.debug('{} because of no data while reading stdout for the server process.'.format(self._crash_message))
                        self._crashed = True
    >               self._output += data
    E               TypeError: can't concat str to bytes
    
    port/server_process.py:307: TypeError

[Radar WebKit Bug Importer]

<rdar://problem/76966005>

[Sam Sneddon [:gsnedders]]

Created attachment 426709 [details]
Patch

[EWS]

Committed r276435 (236898@main): <https://commits.webkit.org/236898@main>

All reviewed patches have been landed. Closing bug and clearing flags on attachment 426709 [details].