gh-127572: Fix `test_structmembers` initialization (GH-127577)

gh-127572: Fix `test_structmembers` initialization.

The 'C' format code expects an `int` as a destination (not a `char`).
This led to test failures on big-endian platforms like s390x. Use the
'c' format code, which expects a `char` as the destination (but requires
a Python byte objects instead of a str).