[3.12] gh-94808: Add test coverage for "starred kind" in _PyPegen_set…

…_expr_context (GH-119222) (GH-119264)

Add test coverage for "starred kind" in _PyPegen_set_expr_context
(cherry picked from commit 8231a24)

Co-authored-by: Mark Jason Dominus (陶敏修) <mjd@pobox.com>
Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>

Lib/test/test_unpack_ex.py

@@ -26,6 +26,12 @@
     >>> a == [7, 8, 9]
     True
 
+Unpack nested implied tuple
+
+    >>> [*[*a]] = [[7,8,9]]
+    >>> a == [[7,8,9]]
+    True
+
 Unpack string... fun!
 
     >>> a, *b = 'one'