Fix type in test_escape test

Fix type in test_escape test

On machines where char is unsigned this could lead to option parsing looping
endlessly. It's also too narrow a type on other hardware.

Found via Tom Lane's monitoring of the buildfarm.

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Security: CVE-2025-1094
Backpatch-through: 13