[3.12] gh-121791: Check for `NULL` in `MethodDescriptor2_new` in `_testcapi` (GH-121792) (#121840)

gh-121791: Check for `NULL` in `MethodDescriptor2_new` in `_testcapi` (GH-121792)
(cherry picked from commit 8b6d4755812d0b02e9f26beb9c9a7714e4c5ac28)

Co-authored-by: sobolevn <mail@sobolevn.me>