Add test for editable option

This test only verifies the option one way: a change inside the
container (a new file) is reflected in the local host repository
outside the container.

A further test where a modification at the host level is reflected in
the container, is still neede.

tests/dockerfile/editable/Dockerfile

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+FROM python:3.5
+
+RUN pip install --no-cache notebook
+
+CMD "/bin/sh"
+
+ADD change.sh /usr/local/bin/change.sh
+ADD verify verify
+
+ARG NB_UID
+ENV HOME /tmp
+WORKDIR ${HOME}
+
+USER $NB_UID

tests/dockerfile/editable/README.rst

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+Docker - Edit mode
+------------------
+
+Using the --editable option with a local repository, one can modify a
+file or create a new file in the container, and this change is
+reflected in the respective host directory. It is essentially a
+shortcut for `--mount
+type=bind,source=<local-host-repository>,target=.` (where the target
+resolves into the container working directory).
+
+This is tested by running the change.sh script inside the container
+(using the 'cmd' argument to the Repo2Docker app), which creates a new
+file, and then verifying on the host side the new file is created with
+the proper contents.
+
+In practice, this can be used to run a notebook from inside a
+container (which provides the proper environment), making changes as
+necessary, which are then immediately reflected in the host
+repository.

tests/dockerfile/editable/change.sh

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+#!/bin/bash
+
+cat <<EOF > newfile
+new contents
+EOF
+exit 0

tests/dockerfile/editable/verify

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+#!/bin/bash
+
+/usr/local/bin/change.sh

tests/test_editable.py

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+import pytest
+import os
+import time
+from conftest import make_test_func
+
+
+DIR = os.path.join(os.path.dirname(__file__), 'dockerfile', 'editable')
+
+
+@pytest.fixture(scope="module")
+def run_repo2docker():
+    def run_test(args):
+        return make_test_func(args)()
+    return run_test
+
+
+def test_editable(run_repo2docker):
+    """Run a local repository in edit mode. Verify a new file has been
+    created afterwards"""
+    newfile = os.path.join(DIR, 'newfile')
+    try:
+        # If the file didn't get properly cleaned up last time, we
+        # need to do that now
+        os.remove(newfile)
+    except FileNotFoundError:
+        pass
+    argv = ['--editable', DIR, '/usr/local/bin/change.sh']
+    run_repo2docker(argv)
+    try:
+        with open(newfile) as fp:
+            contents = fp.read()
+        assert contents == "new contents\n"
+    finally:
+        os.remove(newfile)