establish testing (at least smoke) for the notebooks on the hub instance

with https://github.com/dandi/example-notebooks/issues/9 as the motivator, I think it would be important for us to ensure that notebooks we provide "work" on the hub.

- we need a way to test notebooks on an existing instance of the hub, so we could test new/updated/beta version of the hub
- testing could be just smoke run of the notebook to start with
- I found [this article](https://semaphoreci.com/blog/test-jupyter-notebooks-with-pytest-and-nbmake) which introduces `nbmake`
- running `pytest --nbmake dandi-notebooks/` now, will report below the "results"
- to complete in reasonable time we might want to allow to run them in parallel (might not do parallel on small instance)
- we should be able to trigger "testing" all notebooks for a specific type of the hub instance (is that "scriptable" @satra to start a specific instance and interact with that environment)?
- similarly to https://github.com/dandi/dandi-api-webshots/ and https://github.com/datalad/datalad-extensions/ we could have a repository (e.g. example-notebooks-dashboard) which would contain the README.md which in a table would list which notebooks are ok or failing on which type of hub instance.

current as of 20211027 state of things

```shell
jovyan@jupyter-yarikoptic:~$ pytest --nbmake dandi-notebooks/
===================================== test session starts =====================================
platform linux -- Python 3.9.6, pytest-6.2.5, py-1.10.0, pluggy-1.0.0
rootdir: /home/jovyan
plugins: nbmake-0.10, anyio-3.3.0
collected 9 items

dandi-notebooks/000004_demo_analysis.ipynb F [ 11%]
dandi-notebooks/NWBWidget-demo.ipynb F [ 22%]
dandi-notebooks/Untitled.ipynb . [ 33%]
dandi-notebooks/000055/BruntonLab/peterson21/Fig_coarse_labels.ipynb F [ 44%]
dandi-notebooks/000055/BruntonLab/peterson21/Fig_pow_spectra.ipynb F [ 55%]
dandi-notebooks/000055/BruntonLab/peterson21/Table_coarse_labels.ipynb F [ 66%]
dandi-notebooks/000055/BruntonLab/peterson21/Table_part_characteristics.ipynb F [ 77%]
dandi-notebooks/000055/BruntonLab/peterson21/dashboard.ipynb . [ 88%]
dandi-notebooks/cosyne_2020_tutorial/NWB_tutorial_2019.ipynb . [100%]
```

edit 1:
- as @bendichter [pointed out](https://github.com/dandi/example-notebooks/issues/11#issuecomment-953191468) - some notebooks need environment with nwb extension(s) installed. So we can't just run them all in the same environment. **We might need to annotate and then group and run in specific environments**
- some notebooks have their own `pip install -r requirements.txt` which alters the environment thus making order of execution matter etc. **Not yet sure what to do about that, besides running each test in a new "clone" of the test environment**