Add integration tests!

We run jupyter-repo2docker and verify that the things we
expect to be installed are installed.

tests/conftest.py

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+"""
+Custom test collector for our integration tests.
+
+Each directory that has a script named 'verify' is considered
+a test. jupyter-repo2docker is run on that directory,
+and then ./verify is run inside the built container. It should
+return a non-zero exit code for the test to be considered a
+success.
+"""
+import pytest
+import subprocess
+
+def pytest_collect_file(parent, path):
+    if path.basename == 'verify':
+        return Repo(path, parent)
+
+class Repo(pytest.File):
+    def collect(self):
+        yield RepoTest(self.fspath.basename, self, self.fspath)
+
+
+class RepoTest(pytest.Item):
+    def __init__(self, name, parent, path):
+        super().__init__(name, parent)
+        self.path = path
+
+    def runtest(self):
+        subprocess.check_call([
+            'jupyter-repo2docker',
+            str(self.path.dirname),
+            './verify'
+        ])

tests/venv/numpy/requirements.txt

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+numpy

tests/venv/numpy/verify

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+#!/usr/bin/env python
+import sys
+
+assert sys.version_info[:2] == (3, 5)
+
+import numpy

tests/venv/py2/requirements.txt

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+numpy

tests/venv/py2/runtime.txt

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+python-2.7

tests/venv/py2/verify

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+#!/usr/bin/env python2
+import sys
+
+print(sys.version_info)
+assert sys.version_info[:2] == (2, 7)
+
+import numpy