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/Test.expected

@@ -0,0 +1,2 @@
+| is_int | is_int.py |
+| unreachable_split | unreachable_split.py |

python/ql/test/extractor-tests/splitter/Test.ql

@@ -0,0 +1,4 @@
+import python
+
+from Module m
+select m.getName(), m.getFile().getAbsolutePath()

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

python/ql/test/extractor-tests/splitter/unreachable_split.py

@@ -0,0 +1,13 @@
+#Don't crash looking dominators of post-return part of the expression.
+
+def f(c):
+    return
+    if c:
+        x
+    else:
+        y
+    if c:
+        a
+    else:
+        b
+