supportsOffscreenMsaa property
True, and settable so a test can be the device that answers no —
gfx-20n.
The reason it is worth setting: a frame that stops multisampling because
the device cannot and a frame that stops because something reads the
surface buffer look identical from outside, and
FrameResult.antiAliasing exists to tell them apart. A fake that could
only say yes leaves half of that untested.
Implementation
@override
bool supportsOffscreenMsaa = true;