Test extension loading order

[vidartf]

A remaining task from #56 was to add tests for the intended behavior. This issue tracks that.

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[echarles]

The sort logic is no more in nbclassic and implemented in notebook_shim https://github.com/jupyter/notebook_shim/blob/189915e9ff4b1c926c9439211fdf7f6767c1bb77/notebook_shim/nbserver.py#L84-L92

Should this issue be transferred to notebook_shim?

Also I wonder if the def _link_jupyter_server_extension(serverapp) is even used in notebook_shim as it is normally. a jupyter-server task. Should this be removed?