Testing HTML export

[forsyth2]

CDAT/vcs is disabling sliders to fix a bug where plots do not show up in the exported HTML. This commit automates the relevant test cases.

Testing for disabling sliders (https://github.com/CDAT/vcs/pull/428). This replaces #154. To run:

conda create -n <env_name> -c cdat/label/nightly -c conda-forge jupyterlab jupyterhub "python=3.7" nodejs vcs pip
python -m pip install sidecar
conda activate <env_name>
jupyter labextension install @jupyter-widgets/jupyterlab-manager
jupyter labextension install @jupyter-widgets/jupyterlab-sidecar
jupyter labextension install jupyterlab-tutorial-extension
jupyter labextension install @jupyterlab/hub-extension
npm install
jupyter lab build
conda install -c conda-forge ipywidgets

# To install vcs changes:
cd ../vcs; python setup.py install; cd -

To run tests:
# Close out of other Jupyter Lab instances
jupyter labextension install .
jupyter lab
npx task test -c test_export_plot_html_via_button &> out.txt

[forsyth2]

@downiec @muryanto1 Ready for review.

[muryanto1]

@forsyth2 @downiec your circleci run failed because you have to update .circleci/config.yml in the setup_jupyter_vcdat step, update this line: conda install -n jupyter-vcdat -c cdat/label/v82 -c cdat/label/nightly -c conda-forge vcs 'libnetcdf=4.6.2' testsrunner cdat_info so that it has the following instead: conda install -n jupyter-vcdat -c cdat/label/nightly -c conda-forge vcs testsrunner cdat_info

Above will then let you create the conda env (w/out any conflicts). but even that other testcases may fail due to current existing failures.