Adopt 'src' layout and add 'testing' extras

src/xdist/report.py

@@ -0,0 +1,21 @@
+from __future__ import print_function
+from difflib import unified_diff
+
+
+def report_collection_diff(from_collection, to_collection, from_id, to_id):
+    """Report the collected test difference between two nodes.
+
+    :returns: detailed message describing the difference between the given
+    collections, or None if they are equal.
+    """
+    if from_collection == to_collection:
+        return None
+
+    diff = unified_diff(from_collection, to_collection, fromfile=from_id, tofile=to_id)
+    error_message = (
+        u"Different tests were collected between {from_id} and {to_id}. "
+        u"The difference is:\n"
+        u"{diff}"
+    ).format(from_id=from_id, to_id=to_id, diff="\n".join(diff))
+    msg = "\n".join([x.rstrip() for x in error_message.split("\n")])
+    return msg