Python: Add copy of extractor tests

These get to live next to the existing library and query tests, and are
run as part of both the Python 2 and Python 3 language tests.

python/ql/test/extractor-tests/unicode_decoding/test.py

@@ -0,0 +1,2 @@
+'\\ud835'
+'\ud835'

python/ql/test/extractor-tests/unicode_decoding/test.ql

@@ -0,0 +1,7 @@
+import python
+
+from Object o, string s
+where
+  py_cobjectnames(o, s) and
+  count(string s1 | py_cobjectnames(o, s1)) > 1
+select o, s