Cover the Linux branches in prefs.py that previously had no tests
(font metrics, GPU spoofing, MSAA from profile, canvas noise mask
per renderer, WebGL extension preservation, Xvfb workarounds,
virtual_display no-op) and add construction smoke tests for
_LinuxVirtualDisplay. Also fix two host-platform-dependent tests
so the suite stays green on both Windows and Linux.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>