Set up tests for the extension's Python files. (#4208)

Switch to using pytest.
* __main__.py -> run_all.py (to avoid need for setting PYTHONPATH).

Author: ericsnowcurrently

pythonFiles/tests/__init__.py

@@ -0,0 +1,18 @@
+import os.path
+import sys
+
+import pytest
+
+
+TEST_ROOT = os.path.dirname(__file__)
+SRC_ROOT = os.path.dirname(TEST_ROOT)
+DATASCIENCE_ROOT = os.path.join(SRC_ROOT, 'datascience')
+
+
+if __name__ == '__main__':
+    sys.path.insert(1, DATASCIENCE_ROOT)
+    ec = pytest.main([
+        '--rootdir', SRC_ROOT,
+        TEST_ROOT,
+        ])
+    sys.exit(ec)