Add optional testing requirements

.github/workflows/pipeline.yml

@@ -40,16 +40,20 @@ jobs:
 
     - name: Test Wheel install and import
       run: |
+        mamba create -n testing python==${{ matrix.python-version }}
+        mamba activate testing
         pip install wheel
         which pip
+        which mamba
         python setup.py bdist_wheel
         cd dist
         pip install CADET_Python*.whl
         python -c "import cadet; print(cadet.__version__)"
+        cd ..
     - name: Test with pytest
       run: |
         pip install .[testing]
-        mamba install cadet
-        which cadet-cli
+        mamba install cadet -c conda-forge
         mamba list
+        which cadet-cli
         pytest tests --rootdir=tests -m "not slow"