tests: add test to concurrent write to pipe
tests: add test to concurrent write to pipe

Concurrent write is not supported and should fail. The GIO error is
G_IO_ERROR_PENDING

Fantu/spice-gtk,fgouget/spice-gtk,Fantu/spice-gtk,SPICE/spice-gtk,fgouget/spice-gtk,SPICE/spice-gtk,SPICE/spice-gtk,SPICE/spice-gtk,fgouget/spice-gtk,Fantu/spice-gtk,Fantu/spice-gtk,flexVDI/spice-gtk,flexVDI/spice-gtk