Add regression test of invalid-sequence-index on __getitem__

Ref pylint-dev/astroid#927 (comment)

Author: nelfin

tests/functional/r/regression/regression_dynamic_getitiem.py

@@ -0,0 +1,11 @@
+# pylint: disable=missing-docstring,too-few-public-methods
+
+class DynamicGetitem:
+    def __getitem__(self, key):
+        if key == 'attributes':
+            return []
+        return {'world': 123}
+
+
+ex = DynamicGetitem()
+a = ex['hello']['world']  # false-positive: invalid-sequence-index