Merge bitcoin/bitcoin#22201: test: Fix TestShell to allow running in Jupyter Notebook

168b6c317ca054c1287c36be532964e861f44266 add dummy file param to fix jupyter (Josiah Baker)

Pull request description:

  this fixes argparse to use `parse_known_args`. previously, if an unknown argument was passed, argparse would fail with an `unrecognized arguments: %s` error.

  ## why
  the documentation mentions being able to run `TestShell` in a REPL interpreter or a jupyter notebook. when i tried to run inside a jupyter notebook, i got the following error:

  ![image](https://user-images.githubusercontent.com/7444140/121382910-57554880-c947-11eb-94f2-49da8679528c.png)

  this was due to the notebook passing the filename of the notebook as an argument. this is a known problem with notebooks and argparse, documented here: https://stackoverflow.com/questions/48796169/how-to-fix-ipykernel-launcher-py-error-unrecognized-arguments-in-jupyter

  ## testing
  to test, make sure you have jupyter notebooks installed. you can do this by running:
  ```
  pip install notebook
  ```
  or following instructions from [here](https://jupyterlab.readthedocs.io/en/stable/getting_started/installation.html).

  once installed, start a notebook (`jupyter notebook`), launch a python3 kernel and run the following snippet:

  ```python
  import sys

  # make sure this is the path for your system
  sys.path.insert(0, "/path/to/bitcoin/test/functional")
  from test_framework.test_shell import TestShell

  test = TestShell().setup(num_nodes=2, setup_clean_chain=True)
  ```

  you should see the following output, without errors:
  ![image](https://user-images.githubusercontent.com/7444140/121383301-a307f200-c947-11eb-83b6-6c50b2cada25.png)

  if you are unfamiliar with notebooks, here is a short guide on using them: https://jupyter.readthedocs.io/en/latest/running.html

ACKs for top commit:
  MarcoFalke:
    review ACK 168b6c317ca054c1287c36be532964e861f44266
  jamesob:
    crACK https://github.com/bitcoin/bitcoin/pull/22201/commits/168b6c317ca054c1287c36be532964e861f44266
  practicalswift:
    cr ACK 168b6c317ca054c1287c36be532964e861f44266

Tree-SHA512: 4fee1563bf64a1cf9009934182412446cde03badf2f19553b78ad2cb3ceb0e5e085a5db41ed440473494ac047f04641311ecbba3948761c6553d0ca4b54937b4

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