no cover last server error

we should not need this because this should only raise when a test fails

Author: rmorshea

idom/testing.py

@@ -106,7 +106,7 @@ def open_mount_function(self) -> Iterator[_Mount]:
             yield self._mount
         finally:
             if self.server.last_server_error_for_idom_testing.current is not None:
-                raise self.server.last_server_error_for_idom_testing.current
+                raise self.server.last_server_error_for_idom_testing.current  # pragma: no cover
 
 
 class _RenderServerWithLastError(AnyRenderServer):