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/splitter/is_int.py

@@ -0,0 +1,8 @@
+
+# Handle `is`
+
+def g(c):
+    if c is 3:
+        pass
+    if c is not 4:
+        pass