tests: skip display tests if pipe cannot create offscreen buffers

tests/display/conftest.py

@@ -68,7 +68,9 @@ def gsg(graphics_pipe, graphics_engine):
     )
     graphics_engine.open_windows()
 
-    assert buffer is not None
+    if buffer is None:
+        pytest.skip("GraphicsPipe cannot make offscreen buffers")
+
     yield buffer.gsg
 
     if buffer is not None: