Add regression test of not-callable false-positive

Ref pylint-dev/astroid#946 (comment)

Author: nelfin

tests/functional/r/regression/regression_lambda_inference_not_callable.py

@@ -0,0 +1,7 @@
+# pylint: disable=missing-docstring
+
+data = {
+    'abc': None,
+}
+data['abc'] = lambda: print("Callback called")
+data['abc']()  # false-positive `not-callable`