[Docker] install pytest-xdist for parallel testing

docker/Dockerfile.finn_ci

@@ -61,6 +61,7 @@ RUN git clone --branch $FINN_CI_BRANCH https://github.com/Xilinx/finn /workspace
 RUN pip install -r /workspace/finn/requirements.txt
 RUN apt update; apt install nano
 RUN pip install pytest-dependency
+RUN pip install pytest-xdist
 
 ENV PYTHONPATH "${PYTHONPATH}:/workspace/finn/src"
 ENV PYTHONPATH "${PYTHONPATH}:/workspace/pyverilator"

docker/Dockerfile.finn_dev

@@ -55,6 +55,7 @@ RUN pip install matplotlib
 RUN pip install pytest-dependency
 RUN pip install sphinx
 RUN pip install sphinx_rtd_theme
+RUN pip install pytest-xdist
 
 # switch user
 RUN groupadd -g $GID $GNAME