[ENH] added test params to RNNNetwork on 3429

[julian-fong]

What does this implement/fix? Explain your changes.

Implemented the standard 'get_test_params' class method with the appropriate docstring and applicable parameters.

Added a couple test params for RNNNetwork contributing towards #3429. One test param that covers the default set and another that covers the 'units' parameter.

One thing I noticed when going through the docstring for the RNNNetwork class is that there seems to be a missing reference [1] inside.

This is my first pull request ever so please leave me some feedback! :)

PR checklist

For all contributions

• I've added myself to the list of contributors with any new badges I've earned :-)
  How to: add yourself to the all-contributors file in the sktime root directory (not the CONTRIBUTORS.md). Common badges: code - fixing a bug, or adding code logic. doc - writing or improving documentation or docstrings. bug - reporting or diagnosing a bug (get this plus code if you also fixed the bug in the PR).maintenance - CI, test framework, release.
  See here for full badge reference
• Optionally, I've added myself and possibly others to the CODEOWNERS file - do this if you want to become the owner or maintainer of an estimator you added.
  See here for further details on the algorithm maintainer role.
• The PR title starts with either [ENH], [MNT], [DOC], or [BUG]. [BUG] - bugfix, [MNT] - CI, test framework, [ENH] - adding or improving code, [DOC] - writing or improving documentation or docstrings.

For new estimators

• I've added the estimator to the API reference - in docs/source/api_reference/taskname.rst, follow the pattern.
• I've added one or more illustrative usage examples to the docstring, in a pydocstyle compliant Examples section.
• If the estimator relies on a soft dependency, I've set the python_dependencies tag and ensured
  dependency isolation, see the estimator dependencies guide.

[fkiraly]

Thanks. I approve if tets run.

[fkiraly]

Re missing reference, if you can guess what it should be, adding it would be appreciated.

[fkiraly]

This is my first pull request ever so please leave me some feedback!

All nicely done! PR description is clear, contribution is as specified. You're off to a good start!