Matplotlib tests + helpers methods

* Maplotlib is an important package. It has now its own test checking that a plot can be generated and exported has an image.
* Some helpers methods added to the `Makefile` mainly permitting to clean containers and images
* Improved assertion messages of some tests

scipy-notebook/test/test_matplotlib.py

@@ -0,0 +1,35 @@
+# Copyright (c) Jupyter Development Team.
+# Distributed under the terms of the Modified BSD License.
+
+import logging
+
+import pytest
+import os
+
+LOGGER = logging.getLogger(__name__)
+
+
+def test_matplotlib(container):
+    """Test that matplotlib is able to plot a graph and write it as an image"""
+    host_data_dir = os.path.join(os.path.dirname(os.path.realpath(__file__)), "data")
+    cont_data_dir = "/home/jovyan/data"
+    test_file = "matplotlib_1.py"
+    output_dir = "/tmp"
+    LOGGER.info(f"Test that matplotlib is able to plot a graph and write it as an image ...")
+    command = "sleep infinity"
+    running_container = container.run(
+        volumes={host_data_dir: {"bind": cont_data_dir, "mode": "ro"}},
+        tty=True,
+        command=["start.sh", "bash", "-c", command],
+    )
+    command = f"python {cont_data_dir}/{test_file}"
+    cmd = running_container.exec_run(command)
+    assert cmd.exit_code == 0, f"Command {command} failed"
+    LOGGER.debug(cmd.output.decode("utf-8"))
+    # Checking if the file is generated
+    # https://stackoverflow.com/a/15895594/4413446
+    expected_file = f"{output_dir}/test.png"
+    command = f"test -s {expected_file}"
+    cmd = running_container.exec_run(command)
+    assert cmd.exit_code == 0, f"Command {command} failed"
+    LOGGER.debug(cmd.output.decode("utf-8"))